United States Office of Water EPA 822-B-01-012

Environmental Protection 4304 December 2001

Agency

Ambient Water Quality

Criteria Recommendations

Information Supporting the Development 

of State and Tribal Nutrient Criteria

Rivers and Streams in 

Nutrient Ecoregion I
EPA 822-B-01-012

AMBIENT WATER QUALITY CRITERIA RECOMMENDATIONS

INFORMATION SUPPORTING THE DEVELOPMENT OF STATE AND TRIBAL

NUTRIENT CRITERIA

FOR

RIVERS AND STREAMS IN NUTRIENT ECOREGION I

Willamette and Central Valleys

including all or parts of the States of:

Washington, Oregon, and California,

and the authorized Tribes within the Region

U.S. ENVIRONMENTAL PROTECTION AGENCY

OFFICE OF WATER

OFFICE OF SCIENCE AND TECHNOLOGY

HEALTH AND ECOLOGICAL CRITERIA DIVISION

WASHINGTON, DC

DECEMBER 2001
ii
iii

FOREWORD

This document presents EPA’s nutrient criteria for Rivers and Streams in Nutrient

Ecoregion I. These criteria provide EPA’s recommendations to States and authorized Tribes for

use in establishing their water quality standards consistent with section 303(c) of the Clean

Water Act (CWA). Under section 303(c) of the CWA, States and authorized Tribes have the

primary responsibility for adopting water quality standards as part of State or Tribal law or

regulation. Federal regulations require State and Tribal standards to contain scientifically

defensible water quality criteria that are protective of designated uses. EPA’s recommended

section 304(a) criteria are not laws or regulations; they are guidance that States and Tribes may

use as a starting point in creating their own water quality standards. 

The term “water quality criteria” is used in two sections of the CWA, section 304(a)(1) and

section 303(c)(2). The term has a different impact in each section. On the one hand, in section

304, the term represents a scientific assessment of ecological and human health effects that EPA

recommends to States and authorized Tribes for establishing water quality standards that

ultimately provide a basis for controlling discharges or releases of pollutants or related

parameters. On the other hand, in section 303, ambient water quality criteria are developed by

States and Tribes as part of their water quality standards, to define the level of a pollutant (or in

the case of nutrients, a condition) necessary to protect designated uses in ambient waters. 

 Quantified water quality criteria contained within State or Tribal water quality standards

are essential to a water quality-based approach to pollution control. Whether expressed

numerically or as quantified translations of narrative criteria within State or Tribal water quality

standards, quantified criteria are critical for assessing attainment of designated uses and

measuring progress toward meeting CWA goals.

EPA is developing section 304(a) water quality criteria for nutrients because States and

Tribes consistently identify excessive levels of nutrients as a major reason that as many as half of

the Nation’s surface waters surveyed do not meet water quality objectives, such as full support of

aquatic life. EPA expects to develop nutrient criteria that cover four major types of

waterbodies—lakes and reservoirs, rivers and streams, estuarine and coastal areas, and

wetlands—across 14 major ecoregions of the United States. EPA’s section 304(a) criteria are

intended to provide for the protection and propagation of aquatic life and recreation. To support

the development of nutrient criteria, EPA has published and will continue to publish technical

guidance manuals that describe a process for assessing nutrient conditions in the four waterbody

types listed above. 

EPA’s section 304(a) water quality criteria for nutrients provide numeric water quality

criteria and procedures to help establish quantified criteria within State or Tribal water quality

standards. In the case of nutrients, EPA section 304(a) criteria establish values for causal

variables (e.g., total nitrogen and total phosphorus) and response variables (e.g., turbidity and

chlorophyll a). EPA believes that State and Tribal water quality standards need to include

quantified endpoints for causal and response variables to provide sufficient protection of uses

and to maintain downstream uses. These endpoints will most often be expressed as numeric

water quality criteria or as procedures to translate a State or Tribal narrative criterion into a

quantified endpoint.
iv

States and authorized Tribes have several options in adopting these criteria. EPA

recommends the following approaches, in order of preference:

1. Wherever possible, develop nutrient criteria that fully reflect local conditions and protect

specific designated uses through the process described in EPA’s technical guidance

manuals for nutrient criteria development. Such criteria may be expressed either as

numeric criteria or as procedures to translate a State or Tribal narrative criterion into a

quantified endpoint in State or Tribal water quality standards.

2. Adopt EPA’s section 304(a) water quality criteria for nutrients, either as numeric criteria or

as procedures to translate a State or Tribal narrative nutrient criterion into a quantified

endpoint.

3. Develop nutrient criteria protective of designated uses using other scientifically defensible

methods and appropriate water quality data.

EPA developed the nutrient criteria recommendations in this document with the intent that

they serve as a starting point for States and Tribes to develop more refined criteria, as

appropriate, to reflect local conditions. The values presented in this document generally

represent nutrient levels that protect against the adverse effects of nutrient overenrichment. They

are based on the information that was available to the Agency at the time of this publication. 

EPA expects States and Tribes may have additional information and data that may be utilized in

the refinement of these criteria. EPA offers to work with States and authorized Tribes to

establish the necessary quantitative endpoints to reduce the excess nutrient inputs into our

nation's waters and to prevent any further impairments.

 

Geoffrey H. Grubbs, Director

Office of Science and Technology
v

DISCLAIMER

This document provides technical guidance and recommendations to States, authorized

Tribes, and other authorized jurisdictions to develop water quality criteria and water quality

standards under the Clean Water Act (CWA) to protect against the adverse effects of nutrient

overenrichment. Under the CWA, States and authorized Tribes are to establish water quality

criteria to protect designated uses. State and Tribal decisionmakers retain the discretion to adopt

approaches on a case-by-case basis that differ from this guidance when appropriate and

scientifically defensible. Even though this document contains EPA’s scientific

recommendations regarding ambient concentrations of nutrients that will protect aquatic resource

quality, it does not substitute for the CWA or EPA regulations, nor is it a regulation itself. Thus

it cannot impose legally binding requirements on EPA, States, authorized Tribes, or the regulated

community, and it might not apply to a particular situation or circumstance. EPA may change

this guidance in the future.
vi
vii

EXECUTIVE SUMMARY

Nutrient Program Goals

EPA developed the National Strategy for the Development of Regional Nutrient Criteria

(National Strategy) in June 1998. The strategy presents EPA’s intentions to develop technical

guidance manuals for four types of waters (lakes and reservoirs, rivers and streams, estuaries and

coastal waters, and wetlands) and produce section 304(a) criteria for specific nutrient Ecoregions

by the end of 2000. In addition, the Agency formed Regional Technical Assistance Groups

(RTAGs), which include State and Tribal representatives working to develop more refined and

localized nutrient criteria based on approaches described in the waterbody guidance manuals. 

This document presents EPA’s current recommended criteria for total phosphorus (TP), total

nitrogen (TN), chlorophyll a, and turbidity for rivers and streams in Nutrient Ecoregion I, which

were derived using the procedures described in the Rivers and Streams Nutrient Criteria

Technical Guidance Manual (U.S. EPA, 2000b). 

EPA’s ecoregional nutrient criteria address cultural eutrophication—the adverse effects of

excess human-caused nutrient inputs. The criteria are empirically derived to represent surface

waters that are minimally impacted by human activities and protective of aquatic life and

recreational uses. The information contained in this document represents starting points for

States and Tribes to develop (with assistance from EPA) more refined nutrient criteria.

In developing these criteria recommendations, EPA followed a process that included, to the

extent they were readily available, the following critical elements: 

C Historical and recent nutrient data in Nutrient Ecoregion I. Data sets from Legacy

STORET, NASQAN, NAWQA, and EPA Region 10 were used to assess nutrient

conditions from 1990 to 2000.

C Reference sites/reference conditions in Nutrient Ecoregion I. Reference conditions

presented are based on 25th percentiles of all nutrient data, including a comparison of

reference conditions for the Aggregate Ecoregion versus the subecoregions. States and

Tribes are urged to determine their own reference sites for rivers and streams at different

geographic scales and to compare them to EPA’s reference conditions.

C Models employed for prediction or validation. EPA did not identify any specific models

to develop nutrient criteria. States and Tribes are encouraged to identify and apply

appropriate models to support nutrient criteria development. 

C RTAG expert review and consensus. EPA recommends that when States and Tribes

prepare their nutrient criteria, they obtain the expert review and consent of the RTAG.

C Downstream effects of criteria. EPA encourages the RTAG to assess the potential effects

of the proposed criteria on downstream water quality and uses.
viii

In addition, EPA followed specific QA/QC procedures during data collection and analysis. 

All data were reviewed for duplications. All data were from ambient waters that were not

located directly outside a permitted discharger. The following States indicated that their data

were sampled and analyzed using either standard methods or EPA-approved methods:

Washington, and Oregon. California indicated that standard or EPA-approved methods were

used for some specific nutrient parameters.

The following tables contain a summary of aggregate and level III Ecoregion values for

TN, TP, water column chlorophyll a, and turbidity.

BASED ON 25th PERCENTILES ONLY

Nutrient Parameters Aggregate Nutrient Ecoregion I 

Reference Conditions

Total phosphorus (μg/L) 47

Total nitrogen (mg/L) (reported) 0.31

Chlorophyll a (μg/L) (fluorometric method) 1.8

Turbidity (FTU) 4.25

For subecoregions 3 and 7 the ranges of nutrient parameter reference conditions are as follows:

BASED ON 25th PERCENTILE ONLY

Nutrient Parameters Range of Level III Subecoregions

Reference Conditions

Total phosphorus (μg/L) 40-77

Total nitrogen (mg/L) (reported) 0.32-0.4

Chlorophyll a (μg/L) (fluorometric method) Insufficient data; N/A

Turbidity (FTU) 3.94-7.13
ix

NOTICE OF DOCUMENT AVAILABILITY

This document is available electronically to the public through the Internet at

http://www.epa.gov/OST/standards/nutrient.html. Requests for hard copies of the document

should be made to EPA’s National Service Center for Environmental Publications (NSCEP),

11029 Kenwood Road, Cincinnati, OH 45242; telephone (513) 489-8190 or toll free (800) 490-

9198. Please refer to EPA document number EPA 822-B-01-012.
x

ACKNOWLEDGMENTS

The authors thankfully acknowledge the contributions of the following State and Federal

reviewers: EPA Regions 9 and 10; the States of Washington, Oregon, and California; the Tribes

within the Ecoregion; EPA headquarters personnel from the Office of Wetlands, Oceans, and

Watersheds, Office of Wastewater Management, Office of General Counsel, Office of Research

and Development, and Office of Science and Technology. EPA also acknowledges the external

peer review efforts of Nina Caraco, Institute of Ecosystem Studies; Amy Parker, University of

Georgia; Jan Stevenson, University of Michigan.
xi

TABLE OF CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Notice of Document Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Acknowledgments .............................................................x

List of Tables and Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii

1.0 Introduction ..............................................................1

2.0 Best Use of This Information ................................................6

3.0 Area Covered by This Document .............................................8

3.1 Description of Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Geographical Boundaries of Aggregate Ecoregion I ..........................9

3.3 Level III Subecoregions Within Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Suggested Ecoregional Subdivisions or Adjustments . . . . . . . . . . . . . . . . . . . . . . . . 11

4.0 Data Review for Rivers and Streams in Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . 11

4.1 Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Historical Data from Aggregate Ecoregion I (TP, TN, chl a, turbidity) . . . . . . . . . . . 13

4.3 QA/QC of Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4 Data for All Rivers and Streams Within Aggregate Ecoregion I . . . . . . . . . . . . . . . . 13

4.5 Statistical Analysis of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.6 Classification of River/Stream Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.7 Summary of Data Reduction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.0 Reference Sites and Conditions in Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . . . . . . 24

6.0 Models Used to Predict or Verify Response Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.0 Framework for Refining Recommended Nutrient Criteria for Rivers and Streams in 

Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.1 Example Worksheet for Developing Aggregate Ecoregion and 

Subecoregion Nutrient Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7.2 Setting Seasonal Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.3 When Data/Reference Conditions Are Lacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.4 Site-Specific Criteria Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8.0 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9.0 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

A. Descriptive Statistics Data Tables for Aggregate Ecoregion . . . . . . . . . . . . . . . . . . A-1

B. Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate

 Ecoregion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

C. Quality Control/Quality Assurance Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
xii

LIST OF TABLES AND FIGURES

Tables

Table 1 River and stream records for Aggregate Ecoregion I—Willamette and 

Central Valleys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2 Reference conditions for Aggregate Ecoregion I streams . . . . . . . . . . . . . . . . . 17

Table 3a-b Reference conditions for Ecoregion I streams . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 4 Suggested boundaries for trophic classification of streams from cumulative

frequency distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 5 Nutrient (:g/L) and algal biomass criteria limits recommended to prevent

nuisance conditions and water quality degradation in streams based either on

nutrient-chlorophyll a relationships or preventing risks to stream impairment as

indicated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figures

Figure 1a Fourteen nutrient Ecoregions as delineated by Omernik (2000) ..............4

Figure 1b Level III Ecoregions of the United States ...............................5

Figure 2 Aggregate Ecoregion I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 3 Aggregate Ecoregion I with level III Ecoregions shown . . . . . . . . . . . . . . . . . . 12

Figure 4 Sampling locations within each level III Ecoregion . . . . . . . . . . . . . . . . . . . . . . 14

Figure 5a Illustration of data reduction process for stream data . . . . . . . . . . . . . . . . . . . . . 21

Figure 5b Illustration of reference condition calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1

1.0 INTRODUCTION

Background

Nutrients are essential to the health and diversity of surface waters. However, in excessive

amounts nutrients cause eutrophication or hypereutrophication, which results in overgrowth of

plant life and decline of the biological community. Excessive nutrients can also result in human

health risks, such as the growth of harmful algal blooms, most recently manifested in the

Pfiesteria outbreaks on the Gulf and East Coasts. Chronic nutrient overenrichment of a

waterbody can lead to the following consequences: algal blooms, low dissolved oxygen, fish

kills, overabundance of macrophytes, likely increased sedimentation, and species shifts of both

flora and fauna. 

Historically, National Water Quality Inventories have repeatedly shown that nutrients are a

major cause of ambient water quality use impairments. EPA’s 1996 National Water Quality

Inventory report identifies excessive nutrients as the leading cause of impairment in lakes and

the second leading cause of impairment in rivers (behind siltation). In addition, nutrients were

the second leading cause of impairments after siltation reported by the States in their 1998 lists

of impaired waters. Where use impairment is documented, nutrients contribute roughly 25%-

50% of the impairment nationally. The Clean Water Act (CWA) establishes that, wherever

possible, water quality must provide for the protection and propagation of fish, shellfish, and

wildlife, and recreation in and on the water and/or protecting the physical, chemical, and

biological integrity of those waters. In adopting water quality standards, States and Tribes

designate uses for their waters in consideration of these CWA goals, and establish water quality

criteria that contain sufficient parameters to protect that integrity and those uses. To date, EPA

has not published information and recommendations under section 304(a) for nutrients to assist

States and Tribes in establishing numeric nutrient criteria to protect uses when adopting water

quality standards.

In 1995, EPA gathered a set of national experts and asked them how best to deal with the

national nutrient problem. The experts recommended that the Agency not develop single criteria

values for phosphorus (P) or nitrogen (N) applicable to all waterbodies and regions of the

country. Rather, they recommended that EPA put a premium on regionalization, develop

guidance (assessment tools and control measures) for specific waterbodies and ecological

regions across the country, and use reference conditions (conditions that reflect pristine or

minimally impacted waters) as a basis for developing nutrient criteria.

With these suggestions as starting points, EPA developed the National Strategy for the

Development of Regional Nutrient Criteria (National Strategy), published in June 1998. This

strategy presented EPA’s intentions to develop technical guidance manuals for four types of

waters (lakes and reservoirs, rivers and streams, estuaries and coastal waters, and wetlands), and

thereafter to publish section 304(a) criteria recommendations for specific nutrient Ecoregions. 

Technical guidance manuals for lakes/reservoirs and rivers/streams were published in April 2000

and July 2000, respectively. The technical guidance manual for estuaries/coastal waters was

published in fall 2001, and the draft wetlands technical guidance manual will be published by
2

December 2001. Each manual presents EPA’s recommended approach for developing nutrient

criteria values for a specific waterbody type. In addition, EPA is committed to working with

States and Tribes to develop more refined and localized nutrient criteria based on approaches

described in the waterbody guidance manuals and this document.

Overview of the Nutrient Criteria Development Process

For each nutrient Ecoregion, EPA developed a set of recommendations for two causal

variables (total nitrogen and total phosphorus) and two early indicator response variables

(chlorophyll a [chl a] and some measure of turbidity). Other indicators such as dissolved

oxygen, macrophyte or benthic algal growth or speciation, and other fauna and flora changes are

also useful. However, the first four variables are considered to be the best suited for protecting

designated uses.

The technical guidance manuals describe a process for developing nutrient criteria that

involves consideration of five factors. The first of these is the Regional Technical Assistance

Group (RTAG), which is a body of qualified regional specialists able to objectively evaluate all

of the available evidence and select the value(s) appropriate to nutrient control in the water

bodies of concern. These specialists may come from such disciplines as limnology, biology, or

natural resources management—especially water resource management, chemistry, and ecology. 

The RTAG evaluates and recommends appropriate classification techniques, usually physical,

for criteria determination within an ecoregional construct.

The second factor is the historical information available to establish a perspective of the

resource base. This is usually data and anecdotal information available within the past 10-25

years. This information gives evidence about the background and enrichment trend of the

resource.

The third factor is the existing reference condition, a selection of reference sites chosen to

represent the least culturally impacted waters of the class at the present time. The data from

these sites are combined and a value is selected to represent the reference condition, the best

attainable, most natural condition of the resource base at this time.

The RTAG comprehensively evaluates these three elements to propose a candidate criterion

(initially one each for TP, TN, chl a, and some measure of turbidity).

A fourth factor often employed is mechanistic or empirical models of the historical and

reference condition data to better understand the condition of the resource.

The final element of the process is assessment by the RTAG of the likely downstream

effects of the criterion. Will there be a negative, positive, or neutral effect on the downstream

waterbody? If the RTAG judges that a negative effect is likely, then the proposed State/Tribal

water quality criteria should be revised to ameliorate the potential for any adverse downstream

effects.
3

Although States and authorized Tribes do not necessarily need to incorporate all five

elements into their water quality criteria setting process (e.g., modeling may be significant in

only some instances), the best assurance of a representative and effective criterion is a balanced

incorporation of all five elements.

Because some parts of the country have naturally different soil and parent material nutrient

content, and different precipitation regimes, the application of the criterion development process

should reflect this regional variation. Therefore, an ecoregional approach was chosen. Initially,

the continental United States was divided into 14 separate Ecoregions of similar geographical

characteristics and similar nutrient condition (Figure 1a). Ecoregions are defined as regions of

relative homogeneity in ecological systems; they depict areas within which the mosaic of

ecosystem components (biotic and abiotic as well as terrestrial and aquatic) is different from

adjacent areas in a holistic sense. Geographic characteristics such as soils, vegetation, climate,

geology, and land cover are relatively similar within each Ecoregion (Omernik, 2000). 

The nutrient Ecoregions are aggregates of EPA’s hierarchical level III Ecoregions (see

Figure 1b for map of level III Ecoregions). As such, they are more generalized and less defined

than level III Ecoregions. EPA determined that setting ecoregional criteria for the large-scale

aggregates is not without its drawbacks: variability is high because of the lumping of many

waterbody classes, seasons, and years worth of multipurpose data over a large geographic area. 

For these reasons, the Agency recommends that States and Tribes develop nutrient criteria at the

level III ecoregional scale and at the waterbody-class scale, where those data are readily

available. Data analyses and recommendations on both the large Aggregate Ecoregion scale and

the more refined scales (level III Ecoregions and waterbody classes), where data were available

to make such assessments, are presented for comparison and completeness of analysis. 

Comparison of Nutrient Criteria to Biological Criteria

Biological criteria are quantitative expressions of the desired condition of the aquatic

community. Such criteria can be based on data from sites that represent the least impacted

attainable condition for a particular waterbody type in an Ecoregion, subecoregion, or watershed. 

EPA’s nutrient criteria recommendations and biological criteria recommendations have many

similarities in their basic approaches to development and data requirements. Both are

empirically derived from statistical analysis of field-collected data and expert evaluation of

current reference conditions and historical information. Both use direct measurements from the

environment to integrate the effects of complex processes that vary according to type and

location of waterbody. The resulting criteria recommendations, in both cases, are efficient uses

of existing resources and are holistic indicators of the water quality necessary to protect uses.

States and authorized Tribes can develop and apply nutrient and biological criteria in

tandem, with each providing important and useful information to interpret both the nutrient

enrichment levels and the biological condition of sampled waterbodies. For example, using the

same reference sites for both types of criteria can lead to efficiencies in both sample design and

data analysis. In one effort, environmental managers can obtain information to support

assessment of biological and nutrient condition, either through evaluating existing data sets or 
4

Figure 1a. Fourteen nutrient Ecoregions as delineated by Omernik (2000). Ecoregions were based on ecosystem type, and nutrient conditions.
5

Figure 1b. Level III Ecoregions of the United States.
6

through designing and conducting a common sampling program. The traditional biological

criteria variables of benthic invertebrate and fish sampling can be readily incorporated in a

nutrient assessment. To investigate the effectiveness of this tandem approach, EPA has initiated

pilot projects in both freshwater and marine environments to pursue the relationship between

nutrient overenrichment and apparent declines in diversity of benthic invertebrates and fish.

2.0 BEST USE OF THIS INFORMATION

EPA recommendations published under section 304(a) of the CWA serve several purposes,

including providing guidance to States and Tribes in adopting water quality standards for

nutrients and ultimately controlling discharges or releases of pollutants. The recommendations

also provide guidance to EPA when it determines that it is necessary to promulgate Federal

water quality standards under section 303(c). Other uses include identification of

overenrichment problems, management planning, project evaluation, and determination of status

and trends of water resources. 

State water quality inventories and listings of impaired waters consistently rank nutrient

overenrichment as a top contributor to use impairments. EPA’s water quality standards

regulations at 40 CFR §131.11(a) require States and Tribes to adopt criteria that contain

sufficient parameters and constituents to protect the designated uses of their waters. In addition,

States and Tribes need quantifiable targets for nutrients to assess attainment of uses, develop

water quality-based permit limits and source control plans, and establish targets for total

maximum daily loads (TMDLs).

EPA expects States and Tribes to address nutrient overenrichment in their water quality

standards and to build on existing State and Tribal efforts where possible. States and Tribes can

address nutrient overenrichment through establishment of numerical criteria or use of narrative

criteria statements (e.g., “free from excess nutrients that cause or contribute to undesirable or

nuisance aquatic life or produce adverse physiological response in humans, animals, or plants”). 

In the case of narrative criteria, EPA expects that States and Tribes will establish procedures to

quantitatively translate these statements for both assessment and source control purposes. 

Ecoregional nutrient criteria are developed to represent surface waters that are minimally

impacted by human activities and thus protect against the adverse effects of nutrient

overenrichment from cultural eutrophication. EPA’s recommended process for developing such

criteria includes physical classification of waterbodies, determination of current reference

conditions, evaluation of historical data and other information (such as published literature), use

of models to simulate physical and ecological processes or determine empirical relationships

among causal and response variables (if necessary), expert judgment, and evaluation of

downstream effects. EPA has used elements of this process to produce the information

contained in this document. The causal (total nitrogen, total phosphorus) and biological and

physical response (chlorophyll a, turbidity) variables represent a set of starting points for States

and Tribes to use in establishing their own criteria.
7

EPA recommends that States and Tribes establish numerical criteria based on section

304(a) guidance, section 304(a) guidance modified to reflect site-specific conditions, or other

scientifically defensible methods. For many pollutants, such as toxic chemicals, EPA expects

that section 304(a) guidance will provide an appropriate level of protection without further

modification. EPA has also published methods for modifying 304(a) criteria, such as the water

effect ratio, on a site-specific basis where conditions warrant modification to achieve the

intended level of protection. For nutrients, however, EPA expects that it will usually be

necessary for States and authorized Tribes to be more precise in identifying the nutrient levels

that protect aquatic life and recreational uses. This can be achieved through criteria modified to

reflect a smaller geographic scale than an Ecoregion, such as a subecoregion, the State or Tribe

level, or a specific class of waterbodies. Criteria can be refined by grouping data or performing

analyses at these smaller geographic scales. Refinement can also occur through further

consideration of other elements such as published literature or models.

EPA expects that the values presented in this document generally represent nutrient levels

that protect against the adverse effects of cultural overenrichment and are based on information

available to the Agency at the time of this publication. However, States and Tribes should

critically evaluate this information in light of the specific uses that need to be protected. For

example, more sensitive uses may require more stringent criteria to ensure adequate protection. 

On the other hand, overly stringent levels of protection against cultural eutrophication may

actually fall below the natural load of nutrients for certain waterbodies. In cases such as these,

the level of nutrients specified may not be sufficient to support a productive fishery. In the

criteria derivation process, it is important to distinguish between the natural load associated with

a specific waterbody using historical data and expert judgment and current reference conditions. 

These elements of the criteria derivation process are best addressed by States and Tribes with

access to information and local expertise. Therefore, EPA strongly encourages States and Tribes

to use the information contained in this document to develop more refined criteria according to

the methods described in EPA’s technical guidance manuals for specific waterbody types.

To assist in further refinement of nutrient criteria, EPA has established 10 RTAGs (experts

from EPA Regional Offices and States/Tribes). In refining criteria, States and authorized Tribes

need to provide documentation of data and analyses, along with a defensible rationale, for any

new or revised nutrient criteria they submit to EPA for review and approval. As part of EPA’s

review of State and Tribal standards, EPA intends to seek assurance from the RTAG that

proposed criteria are sufficient to protect uses.

In using the information and recommendations in this document and elsewhere to develop

numerical criteria or procedures to translate narrative criteria, EPA encourages States and Tribes

to:

C Address both chemical causal variables and early indicator response variables. Causal

variables are necessary to protect uses before impairment occurs and to maintain

downstream uses. Early response variables are necessary to warn of possible impairment

and to integrate the effects of variable and potentially unmeasured nutrient loads.
8

C Include variables that can be measured to determine if standards are met, and variables that

can be related to the ultimate sources of excess nutrients.

C Identify appropriate periods of duration (how long) and frequency (how often) of

occurrence in addition to magnitude (how much). EPA does not recommend identifying

nutrient concentrations that must be met at all times; rather a seasonal or annual averaging

period (e.g., based on weekly or biweekly measurements) is considered appropriate. 

However, these central tendency measures should apply each season or each year, except

under the most extraordinary conditions (e.g., a 100-year flood).

3.0 AREA COVERED BY THIS DOCUMENT

This chapter provides a general description of the Aggregate Ecoregion and its

geographical boundaries. Descriptions of the level III subecoregions contained within the

Aggregate Ecoregion are also provided.

3.1 Description of Aggregate Ecoregion I

Ecoregion I is composed of broad, arable, western valleys that are drier, flatter, and much

more densely populated than the neighboring Western Forested Mountains (II). This ecoregion

encompasses two river valley areas, the Willamette Valley in Oregon and Washington and the

Central Valley in California. Soils are typically nutrient-rich and more naturally fertile than

those of the adjacent nutrient regions. They support mostly cropland agriculture. Erosion,

fertilizer use, irrigation return, livestock, urbanization, and industrialization have degraded the

surficial water quality of the region by increasing levels of nutrients, dissolved solids, toxic

compounds, and fecal coliform bacteria. 

The broad, Willamette Valley is composed of nearly level terraces and floodplains that are

interlaced and surrounded by rolling hills. The meandering, low gradient Willamette River and

its tributaries drain the Valley and have formed oxbow lakes. Elevations range from 10 to 1,500

feet. The mean annual precipitation varies from 37 to 60 inches and the average freeze-free

season is 165-210 days. The potential natural vegetation includes Douglas-fir-hemlock-cedar

forests and Oregon oakwoods; in addition, wetlands, Oregon white oak savanna, prairies,

riparian forests of cottonwoods and willows were part of the pre-settlement landscape. Today,

the Willamette Valley is the most important agricultural area in Oregon. Cropland agriculture is

widespread and contrasts with the prevailing land use of the Western Forested Mountains (II). 

The Willamette Valley’s climate is ocean influenced and mild. Precipitation is concentrated in

the fall, winter, and spring months; summers are dry and, correspondingly, summer streamflow

is relatively low. The Valley’s temperate climate and its productive, nutrient-rich soils support

an especially wide range of crops including grass seed vegetables, berries, wine grapes, nursery

stock, Christmas trees, hay, and grain; pastureland is also common. The Valley’s moderate

precipitation and plentiful streamflow furnish enough water for present needs; additional

supplies are available from adjoining mountain ranges. The Willamette Valley is the home to

most of Oregon’s rapidly growing population and industrial base. Urbanization, fertilizer use,

industrialization, irrigation return, nearby logging, and livestock have affected surficial water
9

quality. Dissolved phosphorus in some streams is rising due to human activities including

agricultural use of phosphorus fertilizer, greater runoff from suburban-urban areas, and more

discharge from municipal sewage treatment plants. Dissolved oxygen is decreasing in some

streams within suburbanized and urbanized watersheds. 

The Central Valley of California is composed of the intensively farmed Sacramento and

San Joaquin valleys. Elevations range from 0 to 650 feet. The mean annual precipitation varies

from 5 to 25 inches and the average freeze-free season is 230 to 350 days. The potential natural

vegetation of the Central Valley includes California steppe, tule marshes, and salt bushgreasewood; oak, willow, and cottonwood naturally occurred along streams and salt bush

originally grew on saline sodic soils. The land use mosaic, potential natural vegetation, and

terrain are all different in the nearby Western Forested Mountains (II) and Xeric West (III). 

Long, hot, dry summers and cool, rainy winters are characteristic of the Central Valley. Stream

flow is limited over much of the area during the summer and water for crops often comes from

stream diversions, wells, canals, and reservoirs. More than 90% of the Central Valley is in farms

and ranches; urban or suburban areas have been rapidly expanding but occupy less than 5% of

the Central Valley. Nearly half of the region is in cropland, about three fourths of which is

irrigated. Environmental concerns in the region include high concentrations of salt and toxic

chemicals in drainage waters, high phosphorus and nitrogen concentrations in streams from

nonpoint sources, groundwater contamination from heavy use of agricultural chemicals,

lowering of the groundwater table due to over-pumping, ground subsidence from overdraft

pumping, wildlife habitat loss, and urban sprawl.

The San Joaquin Valley includes some of the most extensively cultivated, irrigated, and

chemically treated land in California; its water quality has been severely affected by land use and

generally deteriorates downstream. Dissolved solid, nitrite plus nitrate, sulfate, and selenium

concentrations have been rising in the San Joaquin Valley as a result of increasing irrigation

return flow and reuse of stream water; runoff from dairies and feedlots has also affected nitrite

and nitrate levels. The California State Water Resources Control Board has declared a 100 mile

segment of the San Joaquin River as “Water Quality Limited.”

In the Sacramento River, median concentrations of most water pollutants are lower than in

the San Joaquin River system. Weathering of metavolcanic and metasedimentary bedrock

containing appreciable concentrations of fixed nitrogen in the Mokelumne River basin has

contributed a significant amount of nitrate to surface waters. 

3.2 Geographical Boundaries of Aggregate Ecoregion I

Ecoregion I is composed of two separate segments that span along the west coast (Figure

2). The small, northern segment (Willamette Valley) begins near the southwestern border of

Washington and continues south into Oregon. The second, larger segment (Central Valley)

begins in north central California and continues south encompassing the middle portion of the

State.
10

Figure 2. Aggregate Ecoregion I.
11

3.3 Level III Subecoregions Within Aggregate Ecoregion I

There are two level III subecoregions contained within Aggregate Ecoregion I (Figure 3). 

The following are brief descriptions provided by Omernik (1999) of the climate, vegetative

cover, topography, and other ecological information pertaining to these subecoregions.

3. Willamette Valley

Rolling prairies, deciduous/coniferous forests, and extensive wetlands characterized the

pre-19th century landscape of this broad, lowland valley. The Willamette Valley is

distinguished from the adjacent Coast Range and Cascades by lower precipitation, less relief,

and a different mosaic of vegetation. Landforms consist of terraces and floodplains, interlaced

and surrounded by rolling hills. Productive soils and a temperate climate make it one of the most

important agricultural areas in Oregon.

7. Central California Valley

Flat, intensively farmed plains with long, hot dry summers and cool wet winters distinguish

the Central California Valley from its neighboring Ecoregions that are either hilly or

mountainous, forest or shrub covered, and generally nonagricultural. Nearly half of the region is

in cropland, about three fourths of which is irrigated. Environmental concerns in the region

include salinity due to evaporation of irrigation water, groundwater contamination from heavy

use of agricultural chemicals, wildlife habitat loss, and urban sprawl.

3.4 Suggested Ecoregional Subdivisions or Adjustments

EPA recommends that the RTAG evaluate the adequacy of EPA nutrient ecoregional and

subecoregional boundaries and refine them as needed to reflect local conditions. See the paper

by Dale Robertson (USGS, 2001b) for an alternative approach to Ecoregions entitled “An

Alternative Regarding the Scheme for Defining Nutrient Criteria for Rivers and Streams.”

4.0 DATA REVIEW FOR RIVERS AND STREAMS IN AGGREGATE ECOREGION I

This section describes the nutrient data EPA has collected and analyzed for this Ecoregion,

including an assessment of data quantity and quality. The data tables present the data for each

causal parameter (total phosphorus and total nitrogen, both reported and calculated from TKN

and nitrite/nitrate), and the primary response variables (some measure of turbidity and

chlorophyll a). EPA considers these parameters essential to nutrient assessment, because the

first two are the main causative agents of enrichment and the two response variables are the early

indicators of enrichment for most surface waters (see Chapter 3 of the Rivers and Streams

Nutrient Criteria Technical Guidance Manual [U.S. EPA, 2000b] for a complete discussion on

choosing causal and response variables).
12

Figure 3. Aggregate Ecoregion I with level III Ecoregions shown.
13

4.1 Data Sources

Data sets from Legacy STORET, NASQAN, NAWQA, and EPA Region 10 were used to

assess nutrient conditions from 1990 to 2000. EPA recommends that the RTAGs identify

additional data sources that can be used to supplement the data sets listed above. In addition, the

RTAGs may utilize published literature values to support quantitative and qualitative analyses.

4.2 Historical Data from Aggregate Ecoregion I (TP, TN, chl a, and turbidity)

EPA recommends that States/Tribes assess long-term trends observed over the past 50

years to assess the relative stability of the systems. This information may be obtained from

scientific literature or documentation of historical trends. To gain additional perspective on

more recent trends, it is recommended that States and Tribes assess nutrient trends over the past

10 years (e.g., what do seasonal variations indicate?).

4.3 QA/QC of Data Sources

An initial quality screen of data was conducted using the rules presented in Appendix C. 

Data remaining after screening for duplications and other QA measures (e.g., poor or unreported

analytical records, sampling errors or omissions, stations associated with outfalls, stormwater

sewers, hazardous waste sites) were used in the statistical analyses.

States within Ecoregion I were contacted regarding the quality of their data and information

on the methods used to sample and analyze their waters. The following States indicated standard

methods or approved EPA methods were used: Washington and Oregon. California indicated

that standard or EPA-approved methods were used for some specific nutrient parameters.

4.4 Data for All Rivers and Streams Within Aggregate Ecoregion I 

Figure 4 shows the location of the sampling stations within each subecoregion. Table 1

presents all data records for all parameters for Aggregate Ecoregion I and subecoregions within

the Aggregate Ecoregion.

4.5 Statistical Analysis of Data

EPA’s Technical Guidance Manual for Developing Nutrient Criteria for Rivers and

Streams describes two ways of establishing a reference condition. One method is to choose the

upper 25th percentile (75th percentile) of a reference population of streams. This is the preferred

method. The 75th percentile is preferred by EPA because it is likely associated with minimally

impacted conditions, will be protective of designated uses, and provides management flexibility. 

When reference streams are not identified, the second method is to determine the lower 25th

percentile of the population of all streams within a region to attempt to approximate the preferred

approach. The 25th percentile of the entire population was chosen by EPA to represent a

surrogate for an actual reference population. Data analyses to date indicate that the lower 25th

percentile from an entire population roughly approximates the 75th percentile for a reference 
14

Figure 4. Sampling locations within each level III Ecoregion.
15

Table 1. River and stream records* for Aggregate Ecoregion I—Willamette and 

Central Valleys

Aggregate Ecoregion

I

Sub ecoR 

3

Sub ecoR 

7

# of named streams 214 171 43

# of stream stations 572 499 73

Key nutrient parameters (listed below)

- # of records for turbidity (all methods) 12,562 11,140 1,422

- # of records for chlorophyll a

(all methods) + periphyton 9,456 8,192 1,264

- # of records for total Kjeldhal nitrogen

(TKN) 14,316 13,070 1,246

- # of records for nitrite + nitrate

(NO2+NO3) 11,641 11,460 181

- # of records for total nitrogen (TN) 292 155 137

- # of records for total phosphorus (TP) 16,179 13,923 2,256

Total # of records for 

key nutrient parameters 64,446 57,940 6,506

*The number of rivers and streams presented in this table is based on the number of rivers and streams for which nutrient data

were provided in the National Nutrient database. This does not imply that this is the total of rivers and streams within the

Ecoregion. States and Tribes should determine the representativeness of the tabular data by comparing this information with any

additional material they may have.

Definitions: (1) # of records refers to the total count of observations for that parameter over the entire decade (1990-1999) for

that particular aggregate or subecoregion. These are counts for all seasons over that decade. (2) # of stream stations refers to

the total number of river and stream stations within the aggregate or subecoregion from which nutrient data was collected. Since

streams and rivers can cross ecoregional boundaries, it is important to note that only those portions of a river or stream (and data

associated with those stations) that exist within the Ecoregion are included within this table.
16

population (see case studies for Minnesota lakes in the Lakes and Reservoirs Nutrient Criteria

Technical Guidance Document [U.S. EPA, 2000a], the case study for Tennessee streams in the

Rivers and Streams Nutrient Criteria Technical Guidance Document [U.S. EPA, 2000b], the

letter from Tennessee Department of Environment and Conservation to Geoffrey Grubbs

[TNDEC, 2000], the unpublished paper entitled “Estimating the Natural Background

Concentrations of Nutrients in Streams and Rivers of the Conterminous United States” [USGS,

2001], and the letter from Matthew Liebman, U.S. EPA Region 1 Nutrient Criteria Coordinator

to Geoffrey Grubbs [U.S. EPA, 2000c]). New York State has also presented evidence that the

25th percentile and the 75th percentile compare well based on user perceptions of water

resources (NYSDEC, 2000).

Tables 2 and 3a-b present potential reference conditions for both the Aggregate Ecoregion

and the subecoregions using both methods. However, the reference stream column is left blank

because EPA does not have reference data and anticipates that States/Tribes will provide

information on reference streams. Tables 3a-b present potential reference conditions for rivers

and streams in the level III subecoregions within the Aggregate Ecoregion. Note that the

footnotes for Table 2 apply to Tables 3a-b. Appendixes A and B provides a complete

presentation of all descriptive statistics for both the Aggregate Ecoregion and the level III

subecoregions.

Tables 4 and 5 are presented for comparison purposes. They allow the reader to determine

where, in the trophic state, the recommended reference conditions fall within traditionally

viewed trophic boundaries.

4.6 Classification of River/Stream Type

Assessing the data by stream type should further reduce the variability in the data analysis. 

There were no readily available classification data in the national datasets used to develop these

criteria. States and Tribes are strongly encouraged to classify their streams before developing a

final criterion.

4.7 Summary of Data Reduction Methods

All descriptive statistics were calculated using the medians for each stream within

Ecoregion I for which data existed. For example, if one stream had 300 observations for

phosphorus over the decade or 1 year’s time, one median resulted. Each median from each

stream was then used in calculating the percentiles for phosphorus for the aggregate nutrient

Ecoregion/subecoregion (level III Ecoregion) by season and year (Figures 5a, 5b).

Preferred Data Choices and Recommendations When Data Are Missing

1. Where data are missing or are very low in total records for a given parameter, use 25th

percentiles for parameters within an adjacent, similar subecoregion within the same

aggregate nutrient Ecoregion, or when a similar subecoregion cannot be determined, use

the 25th percentile for the Aggregate Ecoregion or consider the lowest 25th percentile from 
17

Table 2. Reference conditions for Aggregate Ecoregion I streams

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 127 0.05 3.55 0.23

NO2+NO3-N (mg/L) 88 0.02 8.64 0.15

TN (mg/L) - calculated 0.38

TN (mg/L) - reported 16 0.00 3.05 0.31

TP (μg/L) 178 1 1,900 47

Turbidity (NTU) 32 0.78 34.54 4.38

Turbidity (FTU) 80 0.55 63 4.25

Turbidity (JCU) 1 (z) 45 45 45 (zz)

Chlorophyll a (μg/L) - F 57 0.43 31.10 1.83

Chlorophyll a (μg/L) - S 11 0.9 15.3 1.6

Chlorophyll a (μg/L) - T 1 4.3 4.3 4.3 (zz)

Periphyton Chl a (mg/m2

) 7 63.7 153.8 63.7

* N = largest value reported for a decade/season. TN calculated is based on the sum of TKN+NO2+NO3. TN reported is actual

TN value reported in the database for one sample.

† Median for all seasons’ 25th percentiles, e.g., this value was calculated from four seasons’ 25th percentiles. If the seasonal

25th percentile (P25) TP values are: spring 10 μg/L, summer 15 μg/L, fall 12 μg/L, and winter 5 μg/L, the median value of all

seasons’ P25 will be 11μg/L. 

‡ As determined by the Regional Technical Assistance Groups (RTAGs).

Abbreviations: P25, 25th percentile of all data; P75, 75th percentile of all data; F, Chlorophyll a measured by Fluorometric

method with acid correction; S, Chlorophyll a measured by Spectrophotometric method with acid correction; T, Chlorophyll a b

c measured by Trichromatic method; —, not applicable.

Definitions: (1) Number of Streams refers to the largest number of streams and rivers for which data existed for a given season

within an aggregate nutrient Ecoregion. (2) Medians. All values (min, max, and 25th percentiles) included in the table are

based on waterbody medians. All data for a particular parameter within a stream for the decade were reduced to one median for

that stream. This prevents over-representation of individual waterbodies with a great deal of data versus those with fewer data

points within the statistical analysis. (3) 25th percentile for all seasons is calculated by taking the median of the 4 seasonal 25th

percentiles. If a season is missing, the median was calculated with 3 seasons of data. If fewer than 3 seasons were used to derive

the median, the entry is flagged (z). (4) A 25th percentile for a season is best derived with data from a minimum of 4

streams/season. However, this table provides 25th percentiles that were derived with fewer than 4 streams/season in order to

retain all information for all seasons. In calculating the 25th percentile for a season with fewer than 4 stream medians, the

statistical program automatically used the minimum value within the fewer-than-4 population. If fewer than 4 streams were used

in developing a seasonal quartile and or all-seasons median, the entry is flagged (zz).

Note: For seasonal values, refer to Appendix A, “Descriptive Statistics Data Tables for Aggregate Ecoregion.”
18

Table 3a. Reference conditions for Ecoregion I streams

subecoregion 3

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 96 0.05 2.75 0.21

NO2+NO3-N (mg/L) 85 0.02 8.64 0.15

TN (mg/L) - calculated 0.36

TN (mg/L) - reported 13 0.00 2.99 0.32

TP (μg/L) 138 2 816.25 40

Turbidity (NTU) 31 0.78 34.54 4.66

Turbidity (FTU) 68 0.55 63 3.94

Turbidity (JCU) — — — —

Chlorophyll a (μg/L) - F 57 0.4 31.1 1.8

Chlorophyll a (μg/L) - S — — — —

Chlorophyll a (μg/L) - T 1 4.3 4.3 4.3 (zz)

Periphyton Chl a (mg/m2) 7 63.7 153.8 63.7
19

Table 3b. Reference conditions for Ecoregion I streams

subecoregion 7

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 31 0.05 3.55 0.19

NO2+NO3-N (mg/L) 5 0.11 1.48 0.12

TN (mg/L) - calculated 0.31

TN (mg/L) - reported 3 0.35 2.26 0.35 (zz)

TP (μg/L) 40 11 1,900 77

Turbidity (NTU) 2 (z) 5.20 6.80 5.20 (zz)

Turbidity (FTU) 13 3.23 21 7.13

Turbidity (JCU) 1 (z) 45 45 45 (zz)

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 11 0.9 15.3 1.6

Chlorophyll a (μg/L) - T — — — —

Periphyton Chl a (mg/m2) — — ——

* N = largest value reported for a decade/season. TN calculated is based on the sum of TKN+NO2+NO3. TN reported is actual

TN value reported in the database for one sample.

† Median for all seasons’ 25th percentiles, e.g., this value was calculated from four seasons’ 25th percentiles. If the seasonal

25th percentile (P25) TP values are: spring 10 μg/L, summer 15 μg/L, fall 12 μg/L, and winter 5 μg/L, the median value of all

seasons’ P25 will be 11μg/L. 

‡ As determined by the Regional Technical Assistance Groups (RTAGs).

Abbreviations: P25, 25th percentile of all data; P75, 75th percentile of all data; F, Chlorophyll a measured by Fluorometric

method with acid correction; S, Chlorophyll a measured by Spectrophotometric method with acid correction; T, Chlorophyll a b

c measured by Trichromatic method; —, not applicable.

Definitions: (1) Number of Streams refers to the number of streams and rivers for which data existed for the summer months

since summer is generally when the greatest amount of nutrient sampling is conducted. If another season greatly predominates,

notification is made (s=spring, f=fall, w=winter). (2) Medians. All values (min, max, and 25th percentiles) included in the table

are based on waterbody medians. All data for a particular parameter within a stream for the decade were reduced to one median

for that stream. This prevents over-representation of individual waterbodies with a great deal of data versus those with fewer

data points within the statistical analysis. (3) 25th percentile for all seasons is calculated by taking the median of the 4 seasonal

25th percentiles. If a season is missing, the median was calculated with 3 seasons of data. If fewer than 3 seasons were used to

derive the median, the entry is flagged (z). (4) A 25th percentile for a season is best derived with data from a minimum of 4

streams/season. However, this table provides 25th percentiles that were derived with fewer than 4 streams/season in order to

retain all information for all seasons. In calculating the 25th percentile for a season with fewer than 4 stream medians, the

statistical program automatically used the minimum value within the fewer-than-4 population. If fewer than 4 streams were used

in developing a seasonal quartile and or all-seasons median, the entry is flagged (zz).

Note: For seasonal and yearly values, refer to Appendix B, “Descriptive Statistics Data Tables for Level III Subecoregions

Within Aggregate Ecoregion.”
20

Table 4. Suggested boundaries for trophic classification of streams from cumulative

frequency distributions. The boundary between oligotrophic and mesotrophic systems

represents the lowest third of the distribution and the boundary between mesotrophic and

eutrophic marks the top third of the distribution.

Variable (units)

Oligotrophicmesotrophic 

boundary

Mesotrophic-eutrophic 

boundary

Sample size

(N)

mean benthic chlorophyll (mg m-2)

a 20 70 286

maximum benthic chlorophyll (mg m-2)

a 60 200 176

sestonic chlorophyll 

(:g L-1)

b 10 30 292

TN (:g L-1)

a,c 700 1,500 1,070

TP (:g L-1)

a,b,c 25 75 1,366

Note: This table is provided to allow the reader to make comparisons between the ecoregional criteria provided in this document

and traditional nutrient and biological endpoints.

a

Data from Dodds et al. (1998); bdata from Van Nieuwenhuyse and Jones (1996); cdata from Omernik (1977).

Table 5. Nutrient (μg/L) and algal biomass criteria limits recommended to prevent

nuisance conditions and water quality degradation in streams based either on nutrientchlorophyll a relationships or preventing risks to stream impairment as indicated.

Periphyton Maximum in mg/m2

TN TP DIN SRP Chlorophyll a Impairment Risk Source

100-200 nuisance growth Welch et al. 1988, 1989

275-650 38-90 100-200 nuisance growth Dodds et al. 1997 

 1,500 75 200 eutrophy Dodds et al. 1998

 300 20 150 nuisance growth Clark Fork River Tri-State

Council, MT

 20 Cladophora

nuisance growth

Chetelat et al. 1999

 10-20 Cladophora

nuisance growth

Stevenson unpubl. data

 430 60 eutrophy UK Environ. Agency 1988

 100a 10a 200 nuisance growth Biggs 2000

 25 3 100 reduced invertebrate

diversity

Nordin 1985

15 100 nuisance growth Quinn 1991

1,000 10b ~100 eutrophy Sosiak pers. comm.

Plankton Mean in μg/L

TN TP DIN SRP Chlorophyll a Impairment Risk Source

 300c 42 8 eutrophy Van Nieuwenhuyse and

Jones 1996

70 15 chlorophyll action level OAR 2000

 250c 35 8 eutrophy OECD 1992 (for lakes) a

30-day biomass accrual time.

b

Total dissolved P.

c

Based on Redfield ratio of 7.2N:1P (Smith et al. 1997).
Figure 5a. Illustration of data reduction process for stream data.

21
2

2

Figure 5b. Illustration of reference condition calculation.
23

a subecoregion (level III) within the aggregate nutrient Ecoregion. Without data, one

may assume that the subecoregion in question is as sensitive as the most sensitive

subecoregion within the aggregate.

2. TN calculated: When reported total nitrogen (TN) median values are lacking or very

low in comparison to TKN and Nitrate/Nitrite-N values, the medians for TKN and

nitrite/nitrate-N are added, resulting in a calculated TN value. The number of samples

(N) for calculated TN is not filled in because it is represented by two subsamples of data: 

TKN and nitrite/nitrate-N. Therefore, N/A is placed in this box. 

3. TN reported: This is the median based on reported values for TN from the database.

4. Chlorophyll a: Medians based on all methods are reported; however, the acid-corrected

medians are preferred to the uncorrected medians. In developing a reference condition

from a particular method, it is recommended that the method with the most observations

be used. Fluorometric and spectrophotometric observations are preferred over all other

methods. However, when no data exist for fluorometric and spectrophotometric

methods, trichromatic values may be used. Data from the various techniques are not

interchangeable.

5. Periphyton: Where periphyton data exist, record them separately. For periphytondominated streams, a measure of periphyton chlorophyll is a more appropriate response

variable than planktonic chlorophyll a. See Table 4, page 101, of the Rivers and Streams

Nutrient Technical Guidance Manual (U.S. EPA, 2000b) for values of periphyton and

planktonic chlorophyll a related to eutrophy in streams.

6. Secchi depth: The 75th percentile is reported for Secchi depth because this is the only

variable for which the value of the parameter increases with greater clarity (for lakes and

reservoirs only).

7. Turbidity units: Turbidity units from all methods are reported. FTUs and NTUs are

preferred over JCUs. If FTUs and NTUs do not exist, use JCUs. These units are not

interchangeable. Turbidity is chosen as a response variable in streams because it can be

an indicator of increasing algal biomass due to nutrient enrichment. See pages 32-33 of

the Rivers and Streams Nutrient Technical Guidance Manual for a discussion of turbidity

and correlations with algal growth.

8. Lack of data: A dash (—) represents missing, inadequate, or inconclusive data. 

According to EPA statistical analyses, 5% or fewer of the reported observations are

“below detection.” Because of this low incidence, these data were retained and factored

into the statistical analysis as reported according to the protocols described in Appendix

C, “Quality Control/Quality Assurance Rules.”
24

5.0 REFERENCE SITES AND CONDITIONS IN AGGREGATE ECOREGION I

Reference conditions represent the natural, least impacted conditions, or what is

considered to be the most attainable conditions. This chapter compares the different reference

conditions determined from the two methods and establishes which reference condition is most

appropriate.

• A priori determination of reference sites. The preferred method for establishing reference

condition is to choose the upper percentile of an a priori population of reference streams. 

States and Tribes are encouraged to identify reference conditions based on this method.

• Statistical determination of reference conditions (25th percentile of entire database). See

Tables 2 and 3a-b in Section 4.0.

• RTAG discussion and rationale for selection of reference sites and conditions in

Ecoregion I. The RTAG should compare the results derived from the two methods

described above and present a rationale for the final selection of reference sites. 

6.0 MODELS USED TO PREDICT OR VERIFY RESPONSE PARAMETERS

The RTAG is encouraged to identify and apply relevant models to support nutrient

criteria development. There are three scenarios under which models may be used to derive

criteria or support criteria development:

• Models for predicting correlations between causal and response variables

• Models used to verify reference conditions based on percentiles

• Regression models used to predict reference conditions in impacted areas

Appendix C of the Rivers and Streams Technical Guidance Manual (U.S. EPA, 2000b),

and Chapter 9 of the Lakes and Reservoirs Technical Guidance Manual (U.S. EPA, 2000a)

should be consulted for further details.

7.0 FRAMEWORK FOR REFINING RECOMMENDED NUTRIENT CRITERIA

FOR RIVERS AND STREAMS IN AGGREGATE ECOREGION I

Information on each of the following six weight-of-evidence factors is important to refine

the criteria presented in this document. All elements should be addressed in developing criteria,

as is expressed in EPA’s nutrient criteria technical guidance manuals. It is our expectation that

EPA Regions, States, and Tribes (as RTAGs) will consider these elements as States/Tribes

develop their criteria. This section should be viewed as a worksheet (sections are left blank for

this purpose) to assist in the refinement of nutrient criteria. If many of these elements are

ultimately unaddressed, EPA may rely on the proposed reference conditions presented in Tables
25

3a-b and other literature and information readily available to the EPA Headquarters nutrient

team to develop nutrient water quality recommendations for this Ecoregion.

7.1 Example Worksheet for Developing Aggregate Ecoregion and Subecoregion

Nutrient Criteria

Literature sources: 

 

 

 

 

 

Historical data and trends: 

 

 

 

 

 

Reference condition: 

 

 

 

 

 

Models: 

 

 

 

 

 

RTAG expert review and consensus: 

 

 

 

 

 
26

Downstream effects: 

 

 

 

 

 

7.2 Setting Seasonal Criteria

The recommendations presented in this document are based in part on medians of all the

25th percentile seasonal data (decadal), and as such reflect all seasons and not one particular

season or year. It is recommended that States and Tribes monitor in all seasons to best assess

compliance with the resulting criterion. States/Tribes may choose to develop criteria that reflect

each particular season or given season or a given year when there is significant variability

between seasons/years or designated uses that are specifically tied to one or more seasons of the

year (e.g., recreation, fishing). Using the tables in Appendix A and B, one can set reference

conditions based on a particular season or year and then develop a criterion based on each

individual season. Obviously, this option is season-specific and would require increased

monitoring within each season to assess compliance. If a case can be made that one season is

more appropriate than another season, or more appropriate than the annual median, criteria

should be season specific. For example, in most parts of the country, spring and summer are the

most common growth periods, so criteria for chlorophyll a and Secchi may be set for spring and

summer only. However, caution should be used when developing criteria for TN and TP

because the peak loading of these nutrients may take place in seasons other than summer, such as

winter and spring. For these reasons, EPA developed annual criteria and provided additional

seasonal information in appendices.

7.3 When Data/Reference Conditions Are Lacking

When data are unavailable to develop a reference condition for a particular parameter(s)

within a subecoregion, EPA recommends one of three options: (1) use data from a similar

neighboring subecoregion (e.g., if data are few or nonexistent for the Northern Cascades,

consider using the data and reference conditions developed for the Cascades); (2) use the 25th

percentiles for the Aggregate Ecoregion; or (3) consider using the lowest of the yearly medians

for that parameter calculated for all the subecoregions within the Aggregate Ecoregion.

7.4 Site-Specific Criteria Development 

Criteria may be refined in a number of ways. The best way is to follow the critical

elements of criteria development as well as to refer to the Rivers and Streams Nutrient Criteria

Technical Guidance Manual (U.S. EPA, 2000b). The Technical Guidance Manual presents

sections on each of the following factors to consider in setting criteria:
27

• Refinements to Ecoregions (Section 2.3). See paper by Dale Robertson (USGS, 2001b),

an alternative approach to ecoregions entitled “An Alternative Regarding the Scheme for

Defining Nutrient Criteria for Rivers and Streams.”

• Classification of waterbodies (Chapter 2)

• Setting seasonal criteria to reflect major seasonal climate differences and accounting for

significant or cyclical precipitation events (high-flow/low-flow conditions) (Chapter 4)

8.0 LITERATURE CITED

NYSDEC (New York State Department of Environment and Conservation). 2000. 

Memorandum from Scott Kishbaugh to Jay Bloomfield, September 26, 2000, regarding reference

lakes for nutrient criteria.

Omernik JM. 1999. Primary Distinguishing Characteristics of Level III Ecoregions of the

Continental United States. Draft.

Omernik JM. 2000. Draft Aggregations of Level III Ecoregions for the National Nutrient

Strategy. [http://www.epa.gov/ost/standards/ecomap.htm]

TNDEC (Tennessee Department of Environment and Conservation). 2000. Letter to Geoff

Grubbs, October 5, 2000, containing comments on draft nutrient criteria recommendations.

U.S. EPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs. U.S.

Environmental Protection Agency, Washington, DC. EPA-822-B00-001.

U.S. EPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. U.S.

Environmental Protection Agency, Washington, DC. EPA-822-B00-002.

U.S. EPA. 2000c. Memorandum from Matthew Liebman to Geoffrey Grubbs, December 15,

2000, regarding comments on draft ambient water quality recommendations for development of

numeric nutrient criteria.

USGS (U.S. Geological Survey). 2001a. Unpublished paper title: “Estimating the Natural

Background Concentrations of Nutrients in Streams and Rivers of the Conterminous United

States.” 34 pages.

USGS. 2001b. An Alternative Regarding the Scheme for Defining Nutrient Criteria for Rivers

and Streams. Dale M. Robertson, David A. Saad, and Ann Wieben. Water Resources

Investigations Report 01-4073.

9.0 APPENDICES

A. Descriptive Statistics Data Tables for Aggregate Ecoregion

B. Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ecoregion

C. Quality Control/Quality Assurance Rules
28
APPENDIX A

Descriptive Statistics Data Tables for Aggregate Ecoregion
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Chloro_A_Fluor_cor_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 52 4.65 .40000 51.00 7.12 0.99 153 0.90 1.85 3.10 4.73  SPRING 50 4.81 .40000 23.20 4.93 0.70 103 1.08 1.80 3.35 5.40  SUMMER 57 5.64 .50000 39.00 7.30 0.97 129 0.90 1.89 3.35 5.95  WINTER 17 2.92 .45000 21.30 4.86 1.18 166 0.45 1.20 1.35 2.60  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1998 to 1998 Chloro_A_Peri_Spe_unc_mg_sqm season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 1 153.85 153.85 153.85 . . . 153.85 153.85 153.85 153.85 15 SPRING 1 63.70 63.700 63.70 . . . 63.70 63.70 63.70 63.70 6 SUMMER 7 203.74 50.600 370.60 131.62 49.75 65 50.60 62.00 194.10 370.60 37 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1995 Chloro_A_Phyto_Spec_A_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 11 3.49 .76000 12.47 3.54 1.07 101 0.76 1.39 1.69 5.59  SPRING 11 4.73 1.1650 18.11 5.34 1.61 113 1.17 1.83 2.34 4.83  SUMMER 11 6.48 .99000 26.42 8.94 2.70 138 0.99 2.22 2.51 5.24  WINTER 11 2.07 .32500 9.28 2.61 0.79 126 0.33 0.70 0.99 2.44  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1994 to 1995 Chloro_A_Trich_unco_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 1 4.95 4.9500 4.95 . . . 4.95 4.95 4.95 4.95  SPRING 1 3.70 3.7000 3.70 . . . 3.70 3.70 3.70 3.70  SUMMER 1 7.98 7.9750 7.98 . . . 7.98 7.98 7.98 7.98  WINTER 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 DIP_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 17 156.00 9.5000 1350.00 314.35 76.24 202 9.50 32.50 77.50 107.50 1 SPRING 17 177.12 6.0000 1650.00 384.18 93.18 217 6.00 35.00 95.00 140.00 1 SUMMER 23 126.80 4.0000 995.00 206.56 43.07 163 8.50 27.00 60.00 135.00 3 WINTER 16 405.00 16.250 5250.00 1293.3 323.33 319 16.25 40.00 66.25 132.50 5 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Dissolved_Oxygen_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 110 9.05 1.8000 12.30 1.70 0.16 19 6.75 8.35 9.25 10.10  SPRING 129 9.74 4.5500 13.20 1.66 0.15 17 6.90 8.78 10.00 10.80  SUMMER 127 8.21 .70000 11.20 1.75 0.16 21 5.20 7.20 8.28 9.40  WINTER 91 10.99 6.6000 13.40 1.38 0.15 13 8.45 10.05 10.95 12.20  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Nitrite_Nitrate_NO2_NO3_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 82 0.60 .01000 7.90 1.21 0.13 201 0.02 0.06 0.24 0.50  SPRING 76 0.82 .02000 8.63 1.22 0.14 150 0.06 0.23 0.50 1.06  SUMMER 88 0.62 .01750 8.65 1.30 0.14 211 0.03 0.07 0.24 0.50  WINTER 61 1.44 .02000 9.60 1.65 0.21 114 0.13 0.38 0.83 1.89  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 105 0.48 .05000 3.60 0.56 0.05 116 0.05 0.18 0.36 0.57  SPRING 110 0.50 .05000 3.40 0.49 0.05 98 0.05 0.25 0.38 0.58  SUMMER 127 0.50 .05000 3.50 0.59 0.05 118 0.05 0.20 0.34 0.55  WINTER 83 0.77 .05000 24.00 2.60 0.29 337 0.05 0.25 0.40 0.63  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1992 Phosph_Ortho_Tot_as_P_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 5 59.00 5.0000 140.00 55.05 24.62 93 5.00 30.00 30.00 90.00 14 SPRING 5 86.50 17.500 160.00 60.87 27.22 70 17.50 50.00 65.00 140.00 16 SUMMER 5 55.00 12.500 120.00 42.24 18.89 77 12.50 35.00 35.00 72.50 12 WINTER 4 66.25 20.000 145.00 54.98 27.49 83 20.00 30.00 50.00 102.50 14 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Total_Nitrogen_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 10 0.89 .00000 3.13 0.91 0.29 103 0.00 0.34 0.63 1.28  SPRING 10 0.94 .00000 2.55 0.87 0.27 92 0.00 0.29 0.61 1.61  SUMMER 16 0.63 .00000 2.98 0.86 0.21 137 0.00 0.13 0.30 0.69  WINTER 9 1.39 .32500 3.13 1.07 0.36 77 0.33 0.56 1.14 1.93  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Total_Phosphorus_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 137 129.16 .00000 1450.00 170.24 14.54 132 10.00 45.00 90.00 157.50 3 SPRING 154 143.61 .00000 2350.00 228.51 18.41 159 20.00 50.00 85.00 152.50 3 SUMMER 178 140.36 2.5000 1100.00 174.45 13.08 124 15.00 45.00 90.00 170.00 3 WINTER 111 201.63 2.5000 8350.00 786.19 74.62 390 10.63 50.00 110.00 150.00 3 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Turbidity_FTU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 80 6.86 .30000 95.00 10.87 1.22 159 0.81 2.78 5.00 7.60  SPRING 72 8.41 .60000 19.00 4.33 0.51 51 2.00 5.50 8.00 11.00  SUMMER 80 7.02 .50000 31.00 6.15 0.69 88 1.00 3.00 5.00 8.35  WINTER 61 18.01 1.4000 142.00 20.44 2.62 113 3.00 6.50 13.60 21.00  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1995 to 1995 Turbidity_JCU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  SPRING 1 12.00 12.000 12.00 . . . 12.00 12.00 12.00 12.00 1 WINTER 1 78.00 78.000 78.00 . . . 78.00 78.00 78.00 78.00 7 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Turbidity_NTU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 20 12.73 1.4000 39.33 9.63 2.15 76 1.65 5.45 11.00 18.03  SPRING 17 11.65 .60000 29.75 9.29 2.25 80 0.60 3.30 11.50 15.00  SUMMER 14 8.18 .60000 23.15 6.89 1.84 84 0.60 1.20 7.32 12.80  WINTER 32 18.11 .95000 54.00 15.22 2.69 84 1.93 7.00 10.93 29.00  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Decade and Season from 1992 to 1998 pH_S_U season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 22 7.47 6.9000 7.95 0.27 0.06 4 7.15 7.28 7.46 7.60  SPRING 22 7.49 7.0000 8.15 0.31 0.07 4 7.08 7.20 7.48 7.73  SUMMER 27 7.68 7.1500 8.50 0.33 0.06 4 7.33 7.40 7.63 7.90  WINTER 21 7.37 6.7500 8.03 0.33 0.07 4 6.86 7.20 7.35 7.60  Data were not always available for all years.
APPENDIX B

Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ecoregion
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Chloro_A_Fluor_cor_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 52 4.65 .40000 51.00 7.12 0.99 153 0.90 1.85 3.10 4.73 13 3 SPRING 50 4.81 .40000 23.20 4.93 0.70 103 1.08 1.80 3.35 5.40 17 3 SUMMER 57 5.64 .50000 39.00 7.30 0.97 129 0.90 1.89 3.35 5.95 24 3 WINTER 17 2.92 .45000 21.30 4.86 1.18 166 0.45 1.20 1.35 2.60 21 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1998 to 1998 Chloro_A_Peri_Spe_unc_mg_sqm subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 1 153.85 153.85 153.85 . . . 154 154 154 154 1 3 SPRING 1 63.70 63.700 63.70 . . . 63.7 63.7 63.7 63.7 63 3 SUMMER 7 203.74 50.600 370.60 132 49.7 65 50.6 62.0 194 371 3 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1995 Chloro_A_Phyto_Spec_A_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 7 FALL 11 3.49 .76000 12.47 3.54 1.07 101 0.76 1.39 1.69 5.59 12 7 SPRING 11 4.73 1.1650 18.11 5.34 1.61 113 1.17 1.83 2.34 4.83 18 7 SUMMER 11 6.48 .99000 26.42 8.94 2.70 138 0.99 2.22 2.51 5.24 26 7 WINTER 11 2.07 .32500 9.28 2.61 0.79 126 0.33 0.70 0.99 2.44 9. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1994 to 1995 Chloro_A_Trich_unco_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 1 4.95 4.9500 4.95 . . . 4.95 4.95 4.95 4.95 4. 3 SPRING 1 3.70 3.7000 3.70 . . . 3.70 3.70 3.70 3.70 3. 3 SUMMER 1 7.98 7.9750 7.98 . . . 7.98 7.98 7.98 7.98 7. 3 WINTER 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30 0. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 DIP_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 6 95.33 9.5000 245.00 80.7 32.9 85 9.50 52.5 78.8 108 2 3 SPRING 6 62.67 6.0000 150.00 51.6 21.1 82 6.00 35.0 45.0 95.0 1 3 SUMMER 12 77.63 4.0000 355.00 95.6 27.6 123 4.00 19.0 55.8 95.0 3 3 WINTER 5 71.50 35.000 125.00 37.6 16.8 53 35.0 40.0 65.0 92.5 1 7 FALL 11 189.09 12.500 1350.00 389 117 206 12.5 30.0 77.5 145 13 7 SPRING 11 239.55 12.500 1650.00 472 142 197 12.5 32.5 123 175 16 7 SUMMER 11 180.45 10.000 995.00 279 84.1 155 10.0 27.5 108 193 9 7 WINTER 11 556.59 16.250 5250.00 1558 470 280 16.3 40.0 67.5 148 52 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Dissolved_Oxygen_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 71 9.31 1.8000 12.30 1.95 0.23 21 3.60 8.53 9.80 10.4 11 3 SPRING 89 10.04 4.5500 13.20 1.75 0.19 17 6.40 9.33 10.4 11.1 12 3 SUMMER 89 8.37 .70000 11.20 1.95 0.21 23 4.20 7.30 8.70 9.70 10 3 WINTER 55 11.75 8.6000 13.40 0.97 0.13 8 10.3 11.3 11.9 12.5 13 7 FALL 39 8.57 5.8000 10.30 0.96 0.15 11 7.20 7.90 8.60 9.15 10 7 SPRING 40 9.07 6.9000 12.20 1.20 0.19 13 7.19 8.58 9.05 9.54 11 7 SUMMER 38 7.86 5.9000 10.70 1.09 0.18 14 6.00 7.08 7.99 8.55 9. 7 WINTER 36 9.82 6.6000 11.60 1.09 0.18 11 7.10 9.30 9.98 10.7 11 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 79 0.61 .01000 7.90 1.23 0.14 202 0.02 0.05 0.24 0.50 3. 3 SPRING 73 0.82 .02000 8.63 1.24 0.14 150 0.06 0.23 0.51 1.05 3. 3 SUMMER 85 0.62 .01750 8.65 1.32 0.14 213 0.03 0.07 0.24 0.49 1. 3 WINTER 56 1.35 .02000 9.60 1.54 0.21 114 0.13 0.42 0.83 1.86 3. 7 FALL 3 0.43 .06900 1.15 0.62 0.36 143 0.07 0.07 0.08 1.15 1. 7 SPRING 3 0.64 .14500 1.55 0.79 0.46 123 0.15 0.15 0.23 1.55 1. 7 SUMMER 3 0.55 .08750 1.40 0.74 0.43 136 0.09 0.09 0.15 1.40 1. 7 WINTER 5 2.49 .13000 6.00 2.58 1.15 104 0.13 0.27 1.73 4.30 6. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 75 0.44 .05000 2.48 0.42 0.05 95 0.05 0.20 0.31 0.50 1. 3 SPRING 79 0.42 .05000 3.40 0.43 0.05 103 0.05 0.21 0.35 0.46 1. 3 SUMMER 96 0.39 .05000 3.02 0.41 0.04 104 0.05 0.20 0.30 0.47 0. 3 WINTER 56 0.43 .05000 1.38 0.29 0.04 68 0.05 0.23 0.40 0.51 1. 7 FALL 30 0.59 .05000 3.60 0.82 0.15 139 0.05 0.10 0.40 0.75 3. 7 SPRING 31 0.72 .05000 2.50 0.58 0.10 80 0.05 0.25 0.60 1.03 2. 7 SUMMER 31 0.83 .05000 3.50 0.88 0.16 106 0.05 0.13 0.65 1.15 3. 7 WINTER 27 1.47 .05000 24.00 4.52 0.87 307 0.05 0.30 0.45 1.23 1. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1992 Phosph_Ortho_Tot_as_P_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 2 85.00 30.000 140.00 77.8 55.0 92 30.0 30.0 85.0 140 1 3 SPRING 2 95.00 50.000 140.00 63.6 45.0 67 50.0 50.0 95.0 140 1 3 SUMMER 2 53.75 35.000 72.50 26.5 18.8 49 35.0 35.0 53.8 72.5 72 3 WINTER 1 40.00 40.000 40.00 . . . 40.0 40.0 40.0 40.0 40 7 FALL 3 41.67 5.0000 90.00 43.7 25.2 105 5.00 5.00 30.0 90.0 90 7 SPRING 3 80.83 17.500 160.00 72.6 41.9 90 17.5 17.5 65.0 160 1 7 SUMMER 3 55.83 12.500 120.00 56.7 32.7 102 12.5 12.5 35.0 120 1 7 WINTER 3 75.00 20.000 145.00 63.8 36.9 85 20.0 20.0 60.0 145 1 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Total_Nitrogen_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 7 0.96 .00000 3.13 1.03 0.39 107 0.00 0.38 0.64 1.28 3. 3 SPRING 7 0.91 .00000 2.55 0.90 0.34 100 0.00 0.26 0.67 1.61 2. 3 SUMMER 13 0.52 .00000 2.98 0.80 0.22 153 0.00 0.08 0.24 0.68 2. 3 WINTER 6 1.38 .32500 3.00 0.98 0.40 71 0.33 0.56 1.22 1.93 3. 7 FALL 3 0.73 .29000 1.58 0.73 0.42 99 0.29 0.29 0.34 1.58 1. 7 SPRING 3 1.02 .39000 2.13 0.96 0.55 94 0.39 0.39 0.56 2.13 2. 7 SUMMER 3 1.08 .30000 2.40 1.15 0.66 106 0.30 0.30 0.55 2.40 2. 7 WINTER 3 1.41 .50000 3.13 1.48 0.86 105 0.50 0.50 0.62 3.13 3. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Total_Phosphorus_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 FALL 97 109.85 2.5000 842.50 110 11.1 100 16.3 40.0 82.5 150  3 SPRING 114 111.44 .00000 790.00 130 12.1 116 10.0 50.0 75.0 130  3 SUMMER 138 118.51 2.5000 1050.00 144 12.3 122 11.0 40.0 80.0 150  3 WINTER 73 109.19 2.5000 420.00 83.3 9.74 76 10.6 40.0 100 130  7 FALL 40 176.00 .00000 1450.00 261 41.3 149 4.38 47.5 113 206  7 SPRING 40 235.31 20.000 2350.00 380 60.1 162 20.0 77.5 146 228  7 SUMMER 40 215.75 20.000 1100.00 239 37.9 111 20.0 77.5 138 283  7 WINTER 38 379.21 2.5000 8350.00 1332 216 351 10.0 90.0 140 265  Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Turbidity_FTU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 68 6.71 .30000 95.00 11.7 1.42 174 0.63 2.48 4.64 6.80 17 3 SPRING 60 7.75 .60000 18.50 4.07 0.53 53 1.50 5.13 7.31 10.0 16 3 SUMMER 67 6.07 .50000 31.00 5.66 0.69 93 1.00 2.75 4.50 7.40 16 3 WINTER 49 19.98 1.4000 142.00 22.3 3.18 111 3.00 8.00 16.8 22.5 56 7 FALL 12 7.67 1.6500 15.00 3.80 1.10 50 1.65 5.19 6.88 9.38 15 7 SPRING 12 11.74 3.9000 19.00 4.19 1.21 36 3.90 8.75 12.4 14.4 19 7 SUMMER 13 11.90 2.6000 27.25 6.48 1.80 54 2.60 8.00 11.5 13.8 27 7 WINTER 12 10.00 3.8500 23.00 5.27 1.52 53 3.85 6.25 9.38 12.8 23 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1995 to 1995 Turbidity_JCU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 7 SPRING 1 12.00 12.000 12.00 . . . 12.0 12.0 12.0 12.0 12 7 WINTER 1 78.00 78.000 78.00 . . . 78.0 78.0 78.0 78.0 78 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Turbidity_NTU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 18 13.66 1.4000 39.33 9.70 2.29 71 1.40 6.03 12.8 18.1 39 3 SPRING 17 11.65 .60000 29.75 9.29 2.25 80 0.60 3.30 11.5 15.0 29 3 SUMMER 14 8.18 .60000 23.15 6.89 1.84 84 0.60 1.20 7.32 12.8 23 3 WINTER 31 18.45 .95000 54.00 15.3 2.76 83 1.93 7.00 11.9 29.0 52 7 FALL 2 4.30 2.7000 5.90 2.26 1.60 53 2.70 2.70 4.30 5.90 5. 7 WINTER 1 7.70 7.7000 7.70 . . . 7.70 7.70 7.70 7.70 7. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1992 to 1998 pH_S_U subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 3 FALL 13 7.32 6.9000 7.60 0.20 0.05 3 6.90 7.20 7.31 7.46 7. 3 SPRING 13 7.34 7.0000 7.71 0.22 0.06 3 7.00 7.20 7.29 7.50 7. 3 SUMMER 18 7.60 7.1500 8.50 0.36 0.08 5 7.15 7.35 7.49 7.64 8. 3 WINTER 12 7.19 6.7500 7.50 0.22 0.06 3 6.75 7.13 7.20 7.36 7. 7 FALL 9 7.69 7.4000 7.95 0.20 0.07 3 7.40 7.55 7.78 7.83 7. 7 SPRING 9 7.70 7.2000 8.15 0.32 0.11 4 7.20 7.43 7.80 7.88 8. 7 SUMMER 9 7.85 7.4000 8.10 0.22 0.07 3 7.40 7.75 7.88 8.00 8. 7 WINTER 9 7.61 7.1000 8.03 0.29 0.10 4 7.10 7.43 7.65 7.83 8. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Chloro_A_Fluor_cor_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 29 3.80 .60000 13.40 3.44 0.64 90 0.80 1.40 3.00 4.40  3 1990 SPRING 25 3.21 .50000 12.60 2.94 0.59 92 0.50 1.20 2.15 3.85  3 1990 SUMMER 36 4.19 .30000 25.00 4.48 0.75 107 0.50 1.35 3.00 5.30  3 1990 WINTER 7 1.43 .70000 2.80 0.67 0.25 47 0.70 1.00 1.30 1.60  3 1991 FALL 38 4.01 .30000 28.40 5.16 0.84 129 0.50 1.60 2.05 4.90  3 1991 SPRING 12 3.66 .90000 7.75 2.07 0.60 57 0.90 2.15 3.10 5.00  3 1991 SUMMER 39 4.30 .50000 16.30 3.32 0.53 77 0.90 1.80 3.20 6.30  3 1991 WINTER 6 1.73 1.1500 2.40 0.44 0.18 25 1.15 1.35 1.80 1.90  3 1992 FALL 20 4.70 .50000 14.20 4.62 1.03 98 0.68 1.38 2.35 9.28  3 1992 SPRING 19 7.22 .40000 30.50 8.35 1.91 116 0.40 1.25 3.90 15.7  3 1992 SUMMER 40 7.55 .60000 48.75 10.6 1.68 141 0.90 1.83 3.53 7.75  3 1992 WINTER 6 2.04 1.2000 4.20 1.08 0.44 53 1.20 1.60 1.68 1.90  3 1993 FALL 24 3.62 .65000 14.90 3.63 0.74 100 0.80 1.60 2.10 4.55  3 1993 SPRING 15 3.05 1.6000 5.55 1.51 0.39 49 1.60 1.70 2.00 4.30  3 1993 SUMMER 32 7.26 1.1000 74.00 13.9 2.46 192 1.20 1.33 2.60 5.43  3 1994 FALL 37 8.19 .40000 110.00 18.1 2.97 221 0.80 2.30 3.70 5.90  3 1994 SPRING 23 6.02 .60000 23.20 5.24 1.09 87 0.70 2.50 4.00 8.60  3 1994 SUMMER 37 10.05 1.7000 108.00 19.0 3.12 189 1.75 2.80 4.85 7.30  3 1994 WINTER 15 9.58 1.2000 40.10 12.7 3.28 132 1.20 1.90 3.05 17.4  3 1995 FALL 33 6.37 .60000 32.00 6.31 1.10 99 1.20 3.00 4.80 6.00  3 1995 SPRING 16 4.76 1.2000 15.00 3.10 0.77 65 1.20 3.05 4.45 5.53  3 1995 SUMMER 36 5.84 .70000 34.70 7.46 1.24 128 0.80 2.20 3.13 6.00  3 1995 WINTER 12 0.82 .30000 2.50 0.61 0.18 74 0.30 0.45 0.60 1.00  3 1996 FALL 34 4.20 .40000 12.00 2.74 0.47 65 0.90 2.15 3.93 5.60  3 1996 SPRING 26 5.30 .45000 16.00 4.24 0.83 80 0.95 2.00 3.95 8.05  3 1996 SUMMER 38 7.01 .75000 45.00 9.07 1.47 129 1.00 2.00 3.90 7.70  3 1996 WINTER 7 0.63 .40000 0.80 0.13 0.05 20 0.40 0.60 0.60 0.70  3 1997 FALL 35 6.17 1.0000 70.00 11.4 1.92 184 1.20 2.70 4.20 5.60  3 1997 SPRING 23 6.43 1.2500 24.60 5.56 1.16 86 1.50 2.25 4.90 8.30  3 1997 SUMMER 35 7.18 .60000 39.60 9.57 1.62 133 0.80 1.80 3.00 7.05  3 1998 SPRING 10 4.01 2.0000 7.40 1.40 0.44 35 2.00 3.40 3.90 4.30  3 1998 SUMMER 2 3.25 2.6000 3.90 0.92 0.65 28 2.60 2.60 3.25 3.90 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1998 to 1998 Chloro_A_Peri_Spe_unc_mg_sqm subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 FALL 1 153.85 153.85 153.85 . . . 154 154 154 154  3 1998 SPRING 1 63.70 63.700 63.70 . . . 63.7 63.7 63.7 63.7  3 1998 SUMMER 7 203.74 50.600 370.60 132 49.7 65 50.6 62.0 194 371 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1995 Chloro_A_Phyto_Spec_A_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  7 1990 FALL 11 5.46 .86000 28.55 8.03 2.42 147 0.86 1.72 2.10 7.75  7 1990 SPRING 11 6.64 1.3900 18.83 7.16 2.16 108 1.39 2.01 2.62 16.8  7 1990 SUMMER 11 8.49 .93000 36.43 13.0 3.91 153 0.93 2.40 3.00 4.08  7 1990 WINTER 11 3.94 .77000 15.75 4.94 1.49 125 0.77 1.20 1.68 3.55  7 1991 FALL 11 5.17 .66000 24.95 7.11 2.14 137 0.66 1.74 1.95 5.39  7 1991 SPRING 11 5.91 .68000 34.35 9.98 3.01 169 0.68 1.51 2.35 3.41  7 1991 SUMMER 11 11.96 .91000 69.15 22.4 6.75 187 0.91 1.85 2.37 4.55  7 1991 WINTER 11 6.49 1.0200 38.64 11.1 3.34 170 1.02 1.25 2.27 6.08  7 1992 FALL 11 4.20 .31000 15.84 4.80 1.45 114 0.31 1.64 2.01 5.74  7 1992 SPRING 11 7.06 1.4200 27.68 8.78 2.65 124 1.42 1.83 4.17 5.18  7 1992 SUMMER 11 5.82 .95500 32.87 9.06 2.73 156 0.96 2.32 2.97 4.74  7 1992 WINTER 11 2.05 .26000 8.34 2.52 0.76 123 0.26 0.39 1.05 2.41  7 1993 FALL 11 3.16 .74000 10.58 3.20 0.96 101 0.74 0.99 1.31 5.91  7 1993 SPRING 11 7.12 .54000 23.18 7.67 2.31 108 0.54 2.41 2.92 10.9  7 1993 SUMMER 11 9.28 .51000 24.64 7.97 2.40 86 0.51 2.08 6.16 15.5  7 1993 WINTER 11 2.07 .32000 10.22 2.96 0.89 143 0.32 0.45 0.68 2.47  7 1994 FALL 11 3.14 .52500 14.37 4.02 1.21 128 0.53 0.85 1.55 4.33  7 1994 SPRING 11 3.95 1.2700 11.04 3.66 1.10 93 1.27 1.40 2.02 4.89  7 1994 SUMMER 11 4.74 .82000 19.86 6.74 2.03 142 0.82 1.59 2.07 2.52  7 1994 WINTER 11 1.49 .14000 7.88 2.21 0.67 148 0.14 0.38 0.59 1.83  7 1995 FALL 11 1.78 .96000 3.20 0.75 0.22 42 0.96 1.34 1.49 2.10  7 1995 SPRING 11 2.50 .82000 4.39 1.43 0.43 57 0.82 0.94 3.01 3.94  7 1995 SUMMER 11 2.95 1.0500 7.24 1.96 0.59 66 1.05 1.56 2.32 3.58  7 1995 WINTER 11 0.95 .33000 2.47 0.60 0.18 64 0.33 0.53 0.90 1.00 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1994 to 1995 Chloro_A_Trich_unco_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1994 FALL 1 4.90 4.9000 4.90 . . . 4.90 4.90 4.90 4.90  3 1994 SUMMER 1 11.90 11.900 11.90 . . . 11.9 11.9 11.9 11.9  3 1995 FALL 1 5.00 5.0000 5.00 . . . 5.00 5.00 5.00 5.00  3 1995 SPRING 1 3.70 3.7000 3.70 . . . 3.70 3.70 3.70 3.70  3 1995 SUMMER 1 4.05 4.0500 4.05 . . . 4.05 4.05 4.05 4.05  3 1995 WINTER 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 DIP_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 2 92.50 55.000 130.00 53.0 37.5 57 55.0 55.0 92.5 130  3 1990 SPRING 2 75.00 40.000 110.00 49.5 35.0 66 40.0 40.0 75.0 110  3 1990 SUMMER 2 117.50 65.000 170.00 74.2 52.5 63 65.0 65.0 118 170  3 1990 WINTER 2 67.50 40.000 95.00 38.9 27.5 58 40.0 40.0 67.5 95.0  3 1991 FALL 2 70.00 30.000 110.00 56.6 40.0 81 30.0 30.0 70.0 110  3 1991 SPRING 2 67.50 25.000 110.00 60.1 42.5 89 25.0 25.0 67.5 110  3 1991 SUMMER 2 60.00 50.000 70.00 14.1 10.0 24 50.0 50.0 60.0 70.0  3 1991 WINTER 1 25.00 25.000 25.00 . . . 25.0 25.0 25.0 25.0  3 1992 FALL 2 112.50 55.000 170.00 81.3 57.5 72 55.0 55.0 113 170  3 1992 SPRING 2 95.00 55.000 135.00 56.6 40.0 60 55.0 55.0 95.0 135  3 1992 SUMMER 2 49.50 24.000 75.00 36.1 25.5 73 24.0 24.0 49.5 75.0  3 1992 WINTER 1 50.00 50.000 50.00 . . . 50.0 50.0 50.0 50.0  3 1993 FALL 5 89.00 50.000 155.00 53.7 24.0 60 50.0 50.0 50.0 140  3 1993 SPRING 5 64.00 30.000 140.00 46.3 20.7 72 30.0 30.0 45.0 75.0  3 1993 SUMMER 5 105.00 50.000 240.00 77.0 34.4 73 50.0 65.0 80.0 90.0  3 1993 WINTER 5 103.00 45.000 190.00 70.9 31.7 69 45.0 50.0 60.0 170  3 1994 FALL 5 108.00 60.000 250.00 79.9 35.7 74 60.0 70.0 75.0 85.0  3 1994 SPRING 5 70.00 35.000 185.00 64.5 28.9 92 35.0 40.0 40.0 50.0  3 1994 SUMMER 5 133.00 5.0000 370.00 143 64.0 108 5.00 55.0 80.0 155  3 1994 WINTER 5 70.00 30.000 125.00 38.6 17.2 55 30.0 40.0 65.0 90.0  3 1995 FALL 2 42.50 35.000 50.00 10.6 7.50 25 35.0 35.0 42.5 50.0  3 1995 SPRING 5 71.00 35.000 150.00 47.5 21.2 67 35.0 40.0 50.0 80.0  3 1995 SUMMER 5 123.00 25.000 355.00 135 60.2 109 25.0 35.0 90.0 110  3 1995 WINTER 5 57.00 25.000 110.00 35.1 15.7 62 25.0 35.0 40.0 75.0  3 1996 FALL 1 245.00 245.00 245.00 . . . 245 245 245 245  3 1998 FALL 1 9.50 9.5000 9.50 . . . 9.50 9.50 9.50 9.50  3 1998 SPRING 1 6.00 6.0000 6.00 . . . 6.00 6.00 6.00 6.00  3 1998 SUMMER 7 41.29 4.0000 127.00 43.5 16.4 105 4.00 8.50 27.0 57.5  7 1990 FALL 3 23.33 5.0000 40.00 17.6 10.1 75 5.00 5.00 25.0 40.0  7 1990 SPRING 3 44.17 7.5000 105.00 53.1 30.6 120 7.50 7.50 20.0 105  7 1990 SUMMER 3 50.00 10.000 110.00 52.9 30.6 106 10.0 10.0 30.0 110  7 1990 WINTER 3 68.33 20.000 145.00 67.1 38.8 98 20.0 20.0 40.0 145 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 DIP_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  7 1991 FALL 3 54.17 12.500 120.00 57.7 33.3 106 12.5 12.5 30.0 120  7 1991 SPRING 3 65.00 25.000 115.00 45.8 26.5 71 25.0 25.0 55.0 115  7 1991 SUMMER 3 42.50 12.500 90.00 41.6 24.0 98 12.5 12.5 25.0 90.0  7 1991 WINTER 3 42.50 12.500 80.00 34.4 19.8 81 12.5 12.5 35.0 80.0  7 1992 FALL 3 50.83 12.500 90.00 38.8 22.4 76 12.5 12.5 50.0 90.0  7 1992 SPRING 5 1275.00 5.0000 6100.00 2698 1206 212 5.00 40.0 90.0 140  7 1992 SUMMER 5 364.50 5.0000 1600.00 693 310 190 5.00 12.5 70.0 135  7 1992 WINTER 5 1740.00 30.000 8400.00 3723 1665 214 30.0 65.0 70.0 135  7 1993 FALL 11 168.41 12.500 1100.00 315 94.9 187 12.5 20.0 85.0 175  7 1993 SPRING 11 116.59 12.500 260.00 93.1 28.1 80 12.5 30.0 115 210  7 1993 SUMMER 11 138.18 10.000 635.00 177 53.4 128 10.0 35.0 110 150  7 1993 WINTER 11 549.09 7.5000 5050.00 1495 451 272 7.50 25.0 95.0 210  7 1994 FALL 10 227.00 5.0000 1600.00 486 154 214 5.00 40.0 65.0 115  7 1994 SPRING 11 217.05 12.500 1650.00 478 144 220 12.5 25.0 100 140  7 1994 SUMMER 11 191.82 5.0000 995.00 284 85.6 148 5.00 20.0 100 190  7 1994 WINTER 10 613.50 20.000 5250.00 1631 516 266 20.0 35.0 95.0 150  7 1995 FALL 2 65.00 35.000 95.00 42.4 30.0 65 35.0 35.0 65.0 95.0  7 1995 SPRING 4 313.75 20.000 1070.00 507 254 162 20.0 22.5 82.5 605  7 1995 SUMMER 1 7.50 7.5000 7.50 . . . 7.50 7.50 7.50 7.50  7 1995 WINTER 3 163.33 20.000 410.00 215 124 131 20.0 20.0 60.0 410 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Dissolved_Oxygen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 27 9.65 1.0000 12.40 2.29 0.44 24 6.20 8.25 10.2 11.0  3 1990 SPRING 23 10.54 6.5000 12.30 1.46 0.30 14 7.90 10.1 10.8 11.5  3 1990 SUMMER 30 8.90 4.1000 11.35 1.68 0.31 19 6.50 7.90 9.23 10.4  3 1990 WINTER 27 12.05 10.450 13.40 0.74 0.14 6 10.5 11.6 12.2 12.5  3 1991 FALL 38 9.61 1.9000 13.40 2.58 0.42 27 2.60 8.60 9.80 11.4  3 1991 SPRING 29 11.42 8.9500 13.00 0.99 0.18 9 9.45 11.1 11.3 12.1  3 1991 SUMMER 37 8.93 2.4500 11.65 2.18 0.36 24 3.10 8.20 9.35 10.4  3 1991 WINTER 32 11.68 6.8000 13.85 1.26 0.22 11 9.45 11.3 11.7 12.5  3 1992 FALL 38 9.13 3.5000 11.65 1.79 0.29 20 3.80 8.30 9.65 10.2  3 1992 SPRING 38 10.03 4.5500 12.80 1.67 0.27 17 4.80 9.55 10.3 11.0  3 1992 SUMMER 38 8.72 3.4000 11.60 1.95 0.32 22 3.70 8.00 8.95 10.1  3 1992 WINTER 32 11.51 8.3000 12.80 0.89 0.16 8 10.5 11.1 11.7 12.1  3 1993 FALL 28 9.79 7.7000 11.70 1.09 0.21 11 8.00 9.20 9.60 10.9  3 1993 SPRING 35 11.02 8.5500 13.80 1.18 0.20 11 9.20 10.2 10.9 11.9  3 1993 SUMMER 34 9.40 6.7000 13.00 1.37 0.23 15 7.45 8.50 9.63 10.4  3 1993 WINTER 32 11.88 9.7000 13.20 0.90 0.16 8 9.70 11.5 12.0 12.6  3 1994 FALL 44 9.74 7.0000 12.20 1.28 0.19 13 7.60 8.93 9.70 10.8  3 1994 SPRING 46 10.02 5.4000 14.20 1.70 0.25 17 7.30 8.90 10.2 11.3  3 1994 SUMMER 50 8.41 .70000 12.90 1.95 0.28 23 5.80 7.80 8.70 9.50  3 1994 WINTER 38 11.54 8.6000 13.30 1.16 0.19 10 9.20 10.7 11.7 12.5  3 1995 FALL 34 9.75 4.6000 11.80 1.34 0.23 14 7.85 9.50 10.0 10.4  3 1995 SPRING 33 10.76 9.3000 12.60 0.98 0.17 9 9.40 10.0 10.4 11.5  3 1995 SUMMER 35 8.70 4.2000 10.90 1.51 0.25 17 6.70 7.50 9.10 9.90  3 1995 WINTER 34 11.16 7.9000 13.75 1.17 0.20 11 9.20 10.5 11.2 12.2  3 1996 FALL 39 9.38 3.6000 11.90 1.50 0.24 16 7.23 8.86 9.25 10.2  3 1996 SPRING 50 10.26 5.0000 13.20 1.44 0.20 14 7.60 9.50 10.5 11.1  3 1996 SUMMER 37 8.73 6.3000 14.45 1.53 0.25 18 6.89 7.70 8.45 9.60  3 1996 WINTER 33 11.82 8.9000 13.65 1.03 0.18 9 9.90 11.3 11.9 12.5  3 1997 FALL 33 9.44 5.5500 11.80 1.60 0.28 17 6.58 8.45 10.1 10.4  3 1997 SPRING 35 10.43 6.4000 13.80 1.37 0.23 13 8.20 9.40 10.6 11.2  3 1997 SUMMER 35 8.36 4.8950 11.20 1.62 0.27 19 5.20 7.30 8.50 9.50  3 1997 WINTER 33 12.09 10.300 13.70 0.85 0.15 7 10.4 11.5 12.1 12.7 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Dissolved_Oxygen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 FALL 1 10.40 10.400 10.40 . . . 10.4 10.4 10.4 10.4  3 1998 SPRING 1 11.00 11.000 11.00 . . . 11.0 11.0 11.0 11.0  3 1998 SUMMER 6 9.78 9.3000 10.60 0.53 0.22 5 9.30 9.30 9.65 10.2  3 1998 WINTER 23 11.39 9.6000 13.70 0.93 0.19 8 9.65 11.0 11.5 12.0  7 1990 FALL 12 8.37 7.6000 9.25 0.44 0.13 5 7.60 8.13 8.23 8.68  7 1990 SPRING 12 8.93 8.5000 9.80 0.34 0.10 4 8.50 8.70 8.90 9.00  7 1990 SUMMER 12 7.76 6.2500 9.20 0.81 0.23 10 6.25 7.18 7.88 8.18  7 1990 WINTER 12 10.25 9.1000 11.05 0.62 0.18 6 9.10 9.85 10.1 10.9  7 1991 FALL 12 8.02 7.0000 9.30 0.69 0.20 9 7.00 7.55 8.00 8.45  7 1991 SPRING 12 8.88 7.9000 10.30 0.65 0.19 7 7.90 8.40 8.93 9.15  7 1991 SUMMER 12 7.84 6.6000 9.80 0.82 0.24 10 6.60 7.28 7.90 8.25  7 1991 WINTER 12 10.50 9.3000 11.00 0.43 0.13 4 9.30 10.4 10.6 10.8  7 1992 FALL 11 7.92 6.4000 8.75 0.72 0.22 9 6.40 7.40 7.90 8.50  7 1992 SPRING 16 8.47 5.9500 10.00 0.92 0.23 11 5.95 8.10 8.65 8.85  7 1992 SUMMER 16 7.83 6.2000 13.90 1.77 0.44 23 6.20 6.85 7.78 8.10  7 1992 WINTER 15 9.76 5.4000 11.20 1.41 0.37 14 5.40 9.80 10.0 10.3  7 1993 FALL 29 8.63 6.5000 10.75 0.99 0.18 12 6.90 8.30 8.60 8.90  7 1993 SPRING 32 8.70 6.4000 10.85 1.08 0.19 12 6.50 8.00 8.70 9.50  7 1993 SUMMER 30 7.96 5.8500 9.60 0.97 0.18 12 6.10 7.40 8.10 8.80  7 1993 WINTER 28 9.54 6.6000 11.40 1.03 0.19 11 7.90 8.90 9.73 10.1  7 1994 FALL 31 8.47 6.1000 10.00 0.97 0.17 11 6.20 7.70 8.60 9.20  7 1994 SPRING 29 9.08 7.8000 10.90 0.72 0.13 8 7.95 8.60 9.25 9.40  7 1994 SUMMER 29 7.76 6.4000 9.80 0.94 0.17 12 6.40 6.80 7.90 8.30  7 1994 WINTER 27 10.10 7.7000 11.90 1.04 0.20 10 8.00 9.80 10.0 10.8  7 1995 FALL 11 8.41 7.7000 9.00 0.49 0.15 6 7.70 7.95 8.50 8.90  7 1995 SPRING 15 9.20 7.1000 10.85 1.16 0.30 13 7.10 8.25 9.80 10.2  7 1995 SUMMER 11 7.78 4.6000 9.05 1.27 0.38 16 4.60 7.00 8.30 8.50  7 1995 WINTER 15 9.14 7.1000 10.50 1.07 0.28 12 7.10 8.40 9.30 10.0  7 1996 FALL 9 9.01 5.8000 10.30 1.30 0.43 14 5.80 9.00 9.00 9.80  7 1996 SPRING 9 10.24 6.9000 12.20 1.58 0.53 15 6.90 10.1 10.2 11.1  7 1996 SUMMER 9 8.48 5.9000 10.70 1.70 0.57 20 5.90 7.20 9.10 9.40  7 1996 WINTER 9 9.94 7.1000 11.60 1.54 0.51 16 7.10 8.50 10.6 10.8 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 43 0.73 .00500 5.95 1.33 0.20 182 0.03 0.13 0.25 0.59  3 1990 SPRING 41 0.74 .03000 7.30 1.35 0.21 184 0.04 0.18 0.36 0.54  3 1990 SUMMER 50 0.72 .03000 8.64 1.46 0.21 202 0.03 0.13 0.25 0.48  3 1990 WINTER 30 1.17 .11000 5.60 1.17 0.21 100 0.13 0.48 0.78 1.52  3 1991 FALL 53 0.78 .00500 6.00 1.36 0.19 175 0.02 0.10 0.23 0.56  3 1991 SPRING 45 0.98 .09000 5.50 1.03 0.15 105 0.13 0.30 0.71 1.05  3 1991 SUMMER 47 0.79 .02000 8.95 1.48 0.22 186 0.04 0.19 0.35 0.59  3 1991 WINTER 39 1.21 .10500 5.40 1.07 0.17 88 0.14 0.44 0.87 1.89  3 1992 FALL 55 0.63 .00500 5.65 1.18 0.16 187 0.01 0.05 0.24 0.37  3 1992 SPRING 52 0.84 .02000 9.95 1.49 0.21 179 0.05 0.22 0.46 0.79  3 1992 SUMMER 51 0.68 .02000 8.65 1.43 0.20 209 0.03 0.10 0.26 0.50  3 1992 WINTER 37 1.47 .08000 9.60 1.70 0.28 115 0.14 0.40 1.00 1.99  3 1993 FALL 33 0.90 .02000 7.90 1.69 0.29 188 0.02 0.16 0.23 0.84  3 1993 SPRING 41 0.84 .07000 2.90 0.67 0.10 80 0.13 0.41 0.61 1.10  3 1993 SUMMER 41 0.58 .03000 4.80 0.87 0.14 149 0.03 0.15 0.33 0.53  3 1993 WINTER 34 1.21 .17000 4.00 0.92 0.16 75 0.18 0.66 0.95 1.40  3 1994 FALL 50 0.54 .02000 5.90 0.99 0.14 185 0.02 0.09 0.21 0.43  3 1994 SPRING 41 0.75 .04000 4.30 0.79 0.12 106 0.06 0.28 0.55 0.94  3 1994 SUMMER 48 0.48 .02000 4.95 0.86 0.12 180 0.03 0.10 0.24 0.41  3 1994 WINTER 39 1.18 .02000 3.50 0.93 0.15 79 0.06 0.41 0.91 1.92  3 1995 FALL 40 0.48 .00500 3.40 0.64 0.10 133 0.05 0.19 0.30 0.45  3 1995 SPRING 43 0.73 .03000 3.40 0.72 0.11 99 0.04 0.24 0.58 1.01  3 1995 SUMMER 40 0.51 .02000 5.10 0.84 0.13 162 0.02 0.17 0.36 0.49  3 1995 WINTER 39 1.15 .10500 3.60 0.87 0.14 76 0.11 0.39 0.86 1.74  3 1996 FALL 32 0.57 .02000 6.10 1.27 0.23 223 0.02 0.06 0.17 0.30  3 1996 SPRING 36 0.71 .02500 3.90 0.80 0.13 114 0.04 0.25 0.42 0.98  3 1996 SUMMER 30 0.53 .00500 5.30 1.08 0.20 203 0.01 0.05 0.15 0.45  3 1996 WINTER 36 0.91 .11000 3.05 0.74 0.12 81 0.12 0.38 0.58 1.31  3 1997 FALL 34 0.61 .00500 4.50 0.90 0.15 147 0.01 0.12 0.28 0.74  3 1997 SPRING 36 0.49 .00500 3.10 0.65 0.11 132 0.02 0.15 0.35 0.53  3 1997 SUMMER 36 0.53 .00500 4.40 0.84 0.14 158 0.01 0.11 0.25 0.56  3 1997 WINTER 26 0.69 .04000 3.80 0.80 0.16 116 0.08 0.17 0.40 0.87 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 FALL 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00  3 1998 SPRING 1 0.03 .02800 0.03 . . . 0.03 0.03 0.03 0.03  3 1998 SUMMER 9 1.25 .01600 7.08 2.22 0.74 177 0.02 0.24 0.74 0.88  3 1998 WINTER 23 0.70 .08000 3.10 0.73 0.15 105 0.09 0.19 0.49 1.00  7 1990 FALL 1 0.05 .05000 0.05 . . . 0.05 0.05 0.05 0.05  7 1990 SPRING 2 0.95 .20000 1.70 1.06 0.75 112 0.20 0.20 0.95 1.70  7 1990 SUMMER 1 2.00 2.0000 2.00 . . . 2.00 2.00 2.00 2.00  7 1990 WINTER 5 2.24 .10000 4.40 1.97 0.88 88 0.10 0.20 3.00 3.50  7 1991 FALL 3 0.32 .02350 0.78 0.40 0.23 124 0.02 0.02 0.17 0.78  7 1991 SPRING 3 0.84 .18000 1.85 0.89 0.51 106 0.18 0.18 0.48 1.85  7 1991 SUMMER 3 0.37 .10500 0.86 0.42 0.24 114 0.11 0.11 0.15 0.86  7 1991 WINTER 5 3.18 .13000 7.60 3.20 1.43 101 0.13 0.30 2.75 5.10  7 1992 FALL 3 0.43 .02500 1.20 0.66 0.38 153 0.03 0.03 0.08 1.20  7 1992 SPRING 3 0.39 .09850 0.83 0.38 0.22 97 0.10 0.10 0.26 0.83  7 1992 SUMMER 3 0.40 .08000 0.96 0.48 0.28 121 0.08 0.08 0.16 0.96  7 1992 WINTER 3 0.74 .13000 1.60 0.77 0.44 104 0.13 0.13 0.48 1.60  7 1993 FALL 3 0.47 .06900 1.20 0.63 0.37 135 0.07 0.07 0.14 1.20  7 1993 SPRING 3 0.60 .14500 1.40 0.70 0.40 116 0.15 0.15 0.25 1.40  7 1993 SUMMER 3 0.52 .07150 1.40 0.76 0.44 146 0.07 0.07 0.10 1.40  7 1993 WINTER 2 0.74 .27000 1.20 0.66 0.47 89 0.27 0.27 0.74 1.20  7 1994 FALL 1 1.60 1.6000 1.60 . . . 1.60 1.60 1.60 1.60  7 1994 SPRING 2 0.94 .17000 1.70 1.08 0.77 116 0.17 0.17 0.94 1.70  7 1994 SUMMER 1 1.80 1.8000 1.80 . . . 1.80 1.80 1.80 1.80  7 1994 WINTER 2 1.09 .33000 1.85 1.07 0.76 99 0.33 0.33 1.09 1.85  7 1995 FALL 2 0.60 .09000 1.10 0.71 0.51 120 0.09 0.09 0.60 1.10  7 1995 SPRING 2 0.36 .11000 0.61 0.35 0.25 98 0.11 0.11 0.36 0.61  7 1995 SUMMER 1 0.08 .08000 0.08 . . . 0.08 0.08 0.08 0.08  7 1995 WINTER 1 1.50 1.5000 1.50 . . . 1.50 1.50 1.50 1.50 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 42 0.47 .05000 1.21 0.26 0.04 54 0.20 0.30 0.45 0.63  3 1990 SPRING 40 0.38 .05000 0.91 0.21 0.03 56 0.05 0.22 0.40 0.50  3 1990 SUMMER 48 0.47 .05000 2.85 0.42 0.06 90 0.13 0.20 0.40 0.56  3 1990 WINTER 33 0.44 .05000 1.05 0.27 0.05 63 0.05 0.20 0.40 0.60  3 1991 FALL 50 0.50 .05000 5.10 0.74 0.10 146 0.05 0.20 0.30 0.55  3 1991 SPRING 45 0.43 .05000 1.54 0.31 0.05 72 0.05 0.27 0.40 0.50  3 1991 SUMMER 48 0.52 .05000 3.19 0.54 0.08 104 0.18 0.27 0.36 0.56  3 1991 WINTER 35 0.53 .05000 1.38 0.32 0.05 61 0.05 0.30 0.46 0.70  3 1992 FALL 47 0.50 .05000 2.50 0.51 0.08 102 0.05 0.25 0.40 0.60  3 1992 SPRING 47 0.45 .05000 2.00 0.39 0.06 85 0.05 0.20 0.38 0.60  3 1992 SUMMER 47 0.56 .06000 3.26 0.62 0.09 110 0.13 0.20 0.40 0.60  3 1992 WINTER 33 0.55 .05000 1.80 0.45 0.08 82 0.05 0.26 0.40 0.74  3 1993 FALL 38 0.37 .05000 1.40 0.24 0.04 65 0.05 0.20 0.30 0.46  3 1993 SPRING 44 0.38 .05000 1.40 0.24 0.04 62 0.05 0.22 0.35 0.50  3 1993 SUMMER 46 0.37 .05000 1.80 0.29 0.04 77 0.05 0.20 0.30 0.47  3 1993 WINTER 37 0.43 .05000 1.30 0.30 0.05 70 0.05 0.20 0.35 0.50  3 1994 FALL 49 0.44 .05000 2.00 0.40 0.06 92 0.05 0.20 0.40 0.52  3 1994 SPRING 54 0.44 .05000 3.60 0.63 0.09 145 0.05 0.20 0.30 0.41  3 1994 SUMMER 65 0.35 .05000 1.90 0.27 0.03 78 0.05 0.20 0.30 0.45  3 1994 WINTER 40 0.44 .05000 1.20 0.24 0.04 56 0.05 0.30 0.41 0.57  3 1995 FALL 40 0.43 .05000 1.50 0.25 0.04 57 0.15 0.30 0.40 0.54  3 1995 SPRING 38 0.40 .05000 1.90 0.36 0.06 90 0.05 0.20 0.33 0.40  3 1995 SUMMER 43 0.35 .05000 1.30 0.21 0.03 59 0.05 0.20 0.30 0.48  3 1995 WINTER 38 0.35 .05000 1.20 0.26 0.04 75 0.05 0.18 0.30 0.40  3 1996 FALL 41 0.32 .05000 1.60 0.33 0.05 102 0.05 0.13 0.21 0.44  3 1996 SPRING 37 0.34 .05000 1.10 0.21 0.04 63 0.05 0.20 0.30 0.43  3 1996 SUMMER 40 0.33 .05000 1.40 0.25 0.04 75 0.05 0.20 0.30 0.44  3 1996 WINTER 36 0.34 .05000 0.80 0.17 0.03 51 0.05 0.21 0.38 0.45  3 1997 FALL 36 0.23 .05000 0.90 0.19 0.03 83 0.05 0.05 0.23 0.37  3 1997 SPRING 39 0.26 .05000 1.05 0.19 0.03 73 0.05 0.13 0.21 0.35  3 1997 SUMMER 38 0.32 .05000 1.20 0.22 0.04 70 0.05 0.20 0.29 0.46  3 1997 WINTER 36 0.32 .05000 0.95 0.21 0.03 64 0.05 0.20 0.29 0.43 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 SPRING 12 0.32 .09000 0.54 0.12 0.03 38 0.09 0.24 0.32 0.40  3 1998 SUMMER 6 0.39 .13000 0.52 0.14 0.06 36 0.13 0.33 0.44 0.48  3 1998 WINTER 32 0.27 .05000 0.50 0.15 0.03 56 0.05 0.10 0.30 0.38  7 1990 FALL 3 0.26 .07500 0.40 0.17 0.10 64 0.08 0.08 0.30 0.40  7 1990 SPRING 3 0.38 .30000 0.55 0.14 0.08 38 0.30 0.30 0.30 0.55  7 1990 SUMMER 3 0.67 .30000 1.00 0.35 0.20 53 0.30 0.30 0.70 1.00  7 1990 WINTER 3 0.67 .40000 1.20 0.46 0.27 69 0.40 0.40 0.40 1.20  7 1991 FALL 3 0.48 .12500 1.00 0.46 0.27 97 0.13 0.13 0.30 1.00  7 1991 SPRING 3 0.65 .30000 0.95 0.33 0.19 50 0.30 0.30 0.70 0.95  7 1991 SUMMER 3 0.83 .40000 1.50 0.59 0.34 70 0.40 0.40 0.60 1.50  7 1991 WINTER 3 0.32 .05000 0.70 0.34 0.20 107 0.05 0.05 0.20 0.70  7 1992 FALL 3 0.23 .10000 0.40 0.15 0.09 65 0.10 0.10 0.20 0.40  7 1992 SPRING 7 1.04 .07500 2.50 1.02 0.39 98 0.08 0.30 0.70 2.50  7 1992 SUMMER 7 0.99 .05000 2.20 0.88 0.33 89 0.05 0.08 0.80 2.10  7 1992 WINTER 6 4.32 .12500 24.00 9.64 3.94 223 0.13 0.40 0.40 0.60  7 1993 FALL 20 0.62 .05000 2.40 0.69 0.15 111 0.05 0.10 0.38 0.80  7 1993 SPRING 23 0.94 .07500 2.10 0.60 0.12 64 0.30 0.40 0.85 1.45  7 1993 SUMMER 23 0.92 .05000 3.50 0.96 0.20 104 0.08 0.20 0.70 1.05  7 1993 WINTER 19 2.31 .10000 28.50 6.36 1.46 275 0.10 0.60 0.90 1.40  7 1994 FALL 22 0.81 .05000 4.80 1.18 0.25 146 0.05 0.18 0.45 0.80  7 1994 SPRING 20 0.99 .07500 7.55 1.60 0.36 162 0.09 0.25 0.80 0.95  7 1994 SUMMER 20 1.10 .05000 3.50 1.01 0.23 91 0.08 0.30 0.85 1.50  7 1994 WINTER 18 1.66 .12500 19.00 4.35 1.03 263 0.13 0.20 0.53 1.05  7 1995 FALL 2 0.35 .20000 0.50 0.21 0.15 61 0.20 0.20 0.35 0.50  7 1995 SPRING 6 1.04 .10000 5.05 1.97 0.80 190 0.10 0.13 0.23 0.50  7 1995 SUMMER 2 0.14 .07500 0.20 0.09 0.06 64 0.08 0.08 0.14 0.20  7 1995 WINTER 5 0.72 .20000 2.20 0.84 0.38 117 0.20 0.20 0.50 0.50  7 1996 FALL 9 0.21 .05000 0.60 0.25 0.08 118 0.05 0.05 0.05 0.40  7 1996 SPRING 9 0.30 .05000 0.90 0.28 0.09 93 0.05 0.20 0.20 0.28  7 1996 SUMMER 9 0.30 .05000 0.90 0.36 0.12 121 0.05 0.05 0.05 0.60  7 1996 WINTER 9 0.40 .05000 1.30 0.46 0.15 114 0.05 0.05 0.30 0.40 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1992 Phosph_Ortho_Tot_as_P_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 2 85.00 30.000 140.00 77.8 55.0 92 30.0 30.0 85.0 140  3 1991 FALL 2 70.00 20.000 120.00 70.7 50.0 101 20.0 20.0 70.0 120  3 1991 SPRING 2 80.00 40.000 120.00 56.6 40.0 71 40.0 40.0 80.0 120  3 1991 SUMMER 2 50.00 35.000 65.00 21.2 15.0 42 35.0 35.0 50.0 65.0  3 1991 WINTER 1 30.00 30.000 30.00 . . . 30.0 30.0 30.0 30.0  3 1992 FALL 2 115.00 50.000 180.00 91.9 65.0 80 50.0 50.0 115 180  3 1992 SPRING 2 110.00 60.000 160.00 70.7 50.0 64 60.0 60.0 110 160  3 1992 SUMMER 2 57.50 35.000 80.00 31.8 22.5 55 35.0 35.0 57.5 80.0  3 1992 WINTER 1 50.00 50.000 50.00 . . . 50.0 50.0 50.0 50.0  7 1990 FALL 1 5.00 5.0000 5.00 . . . 5.00 5.00 5.00 5.00  7 1990 WINTER 2 40.00 20.000 60.00 28.3 20.0 71 20.0 20.0 40.0 60.0  7 1991 FALL 3 48.33 5.0000 120.00 62.5 36.1 129 5.00 5.00 20.0 120  7 1991 SPRING 3 73.33 30.000 130.00 51.3 29.6 70 30.0 30.0 60.0 130  7 1991 SUMMER 3 65.00 5.0000 160.00 83.2 48.0 128 5.00 5.00 30.0 160  7 1991 WINTER 3 60.00 20.000 120.00 52.9 30.6 88 20.0 20.0 40.0 120  7 1992 FALL 3 45.00 5.0000 90.00 42.7 24.7 95 5.00 5.00 40.0 90.0  7 1992 SPRING 3 88.33 5.0000 190.00 93.9 54.2 106 5.00 5.00 70.0 190  7 1992 SUMMER 3 46.67 20.000 80.00 30.6 17.6 65 20.0 20.0 40.0 80.0  7 1992 WINTER 3 83.33 10.000 170.00 80.8 46.7 97 10.0 10.0 70.0 170 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Total_Nitrogen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 2 1.20 .70000 1.70 0.71 0.50 59 0.70 0.70 1.20 1.70  3 1990 SPRING 2 1.58 .45000 2.70 1.59 1.13 101 0.45 0.45 1.58 2.70  3 1990 SUMMER 2 2.00 .60000 3.40 1.98 1.40 99 0.60 0.60 2.00 3.40  3 1990 WINTER 2 1.85 1.5000 2.20 0.49 0.35 27 1.50 1.50 1.85 2.20  3 1991 FALL 1 4.20 4.2000 4.20 . . . 4.20 4.20 4.20 4.20  3 1991 SPRING 2 1.96 1.2100 2.70 1.05 0.75 54 1.21 1.21 1.96 2.70  3 1991 SUMMER 2 1.92 .79500 3.05 1.59 1.13 83 0.80 0.80 1.92 3.05  3 1992 FALL 2 2.39 .57000 4.20 2.57 1.82 108 0.57 0.57 2.39 4.20  3 1992 SPRING 1 2.70 2.7000 2.70 . . . 2.70 2.70 2.70 2.70  3 1992 SUMMER 2 1.97 .74000 3.20 1.74 1.23 88 0.74 0.74 1.97 3.20  3 1992 WINTER 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20  3 1993 FALL 2 2.03 .50000 3.55 2.16 1.53 107 0.50 0.50 2.03 3.55  3 1993 SPRING 2 1.31 .92000 1.70 0.55 0.39 42 0.92 0.92 1.31 1.70  3 1993 SUMMER 2 1.67 .73000 2.60 1.32 0.94 79 0.73 0.73 1.67 2.60  3 1993 WINTER 2 2.06 1.1250 3.00 1.33 0.94 64 1.13 1.13 2.06 3.00  3 1994 FALL 6 1.14 .46000 1.90 0.63 0.26 55 0.46 0.54 1.08 1.77  3 1994 SPRING 2 1.69 .97000 2.40 1.01 0.72 60 0.97 0.97 1.69 2.40  3 1994 SUMMER 2 1.17 .63000 1.70 0.76 0.54 65 0.63 0.63 1.17 1.70  3 1994 WINTER 6 1.61 .32000 3.50 1.20 0.49 74 0.32 0.59 1.38 2.49  3 1995 FALL 5 1.04 .38000 2.70 0.94 0.42 90 0.38 0.55 0.78 0.80  3 1995 SPRING 6 1.00 .26000 2.00 0.72 0.29 72 0.26 0.28 0.94 1.61  3 1995 SUMMER 6 0.93 .26000 2.90 1.00 0.41 107 0.26 0.30 0.59 0.92  3 1995 WINTER 4 0.77 .33000 1.36 0.47 0.23 61 0.33 0.40 0.69 1.14  3 1996 FALL 1 0.22 .22000 0.22 . . . 0.22 0.22 0.22 0.22  3 1996 SPRING 1 0.29 .29000 0.29 . . . 0.29 0.29 0.29 0.29  3 1996 SUMMER 1 0.21 .21000 0.21 . . . 0.21 0.21 0.21 0.21  3 1996 WINTER 1 0.56 .56000 0.56 . . . 0.56 0.56 0.56 0.56  3 1998 FALL 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00  3 1998 SPRING 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00  3 1998 SUMMER 7 0.16 .00000 0.68 0.24 0.09 152 0.00 0.00 0.08 0.18  7 1990 FALL 2 0.63 .40000 0.85 0.32 0.23 51 0.40 0.40 0.63 0.85  7 1990 SPRING 3 1.02 .30000 2.25 1.07 0.62 106 0.30 0.30 0.50 2.25 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Total_Nitrogen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  7 1990 SUMMER 3 1.33 .30000 3.00 1.46 0.84 109 0.30 0.30 0.70 3.00  7 1990 WINTER 3 1.77 .50000 4.20 2.11 1.22 119 0.50 0.50 0.60 4.20  7 1991 FALL 3 0.83 .27000 1.75 0.80 0.46 97 0.27 0.27 0.47 1.75  7 1991 SPRING 3 1.48 .48000 2.75 1.16 0.67 79 0.48 0.48 1.20 2.75  7 1991 SUMMER 3 1.22 .55000 2.40 1.03 0.59 84 0.55 0.55 0.71 2.40  7 1991 WINTER 2 2.00 .50000 3.50 2.12 1.50 106 0.50 0.50 2.00 3.50  7 1992 FALL 2 0.83 .20000 1.45 0.88 0.63 107 0.20 0.20 0.83 1.45  7 1992 SPRING 2 0.93 .56000 1.30 0.52 0.37 56 0.56 0.56 0.93 1.30  7 1992 SUMMER 1 1.75 1.7500 1.75 . . . 1.75 1.75 1.75 1.75  7 1992 WINTER 3 1.24 .53000 2.30 0.94 0.54 75 0.53 0.53 0.89 2.30  7 1993 FALL 1 1.55 1.5500 1.55 . . . 1.55 1.55 1.55 1.55  7 1993 SPRING 2 1.28 .55000 2.00 1.03 0.73 80 0.55 0.55 1.28 2.00  7 1993 SUMMER 1 1.70 1.7000 1.70 . . . 1.70 1.70 1.70 1.70  7 1993 WINTER 2 1.66 .57000 2.75 1.54 1.09 93 0.57 0.57 1.66 2.75  7 1994 FALL 1 2.00 2.0000 2.00 . . . 2.00 2.00 2.00 2.00  7 1994 SPRING 1 2.60 2.6000 2.60 . . . 2.60 2.60 2.60 2.60  7 1994 SUMMER 3 1.03 .20000 2.60 1.36 0.78 131 0.20 0.20 0.30 2.60  7 1994 WINTER 2 1.57 .63000 2.50 1.32 0.94 84 0.63 0.63 1.57 2.50  7 1995 FALL 2 0.95 .29000 1.60 0.93 0.66 98 0.29 0.29 0.95 1.60  7 1995 SPRING 1 1.40 1.4000 1.40 . . . 1.40 1.40 1.40 1.40  7 1995 WINTER 1 3.50 3.5000 3.50 . . . 3.50 3.50 3.50 3.50 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 43 135.87 7.5000 460.00 95.4 14.5 70 30.0 60.0 110 190  3 1990 SPRING 41 108.96 2.5000 340.00 75.5 11.8 69 20.0 50.0 100 140  3 1990 SUMMER 46 120.22 10.000 385.00 85.9 12.7 71 20.0 50.0 110 170  3 1990 WINTER 34 125.29 10.000 420.00 100 17.2 80 30.0 50.0 87.5 200  3 1991 FALL 56 133.39 10.000 685.00 143 19.1 107 20.0 40.0 82.5 165  3 1991 SPRING 48 118.59 2.5000 610.00 100 14.5 85 20.0 50.0 100 145  3 1991 SUMMER 78 155.16 2.5000 950.00 186 21.1 120 20.0 50.0 85.0 180  3 1991 WINTER 40 144.13 10.000 455.00 105 16.6 73 17.5 60.0 130 208  3 1992 FALL 56 130.00 2.5000 1000.00 161 21.5 124 20.0 40.0 87.5 135  3 1992 SPRING 54 138.89 10.000 1300.00 201 27.4 145 20.0 50.0 82.5 145  3 1992 SUMMER 54 138.66 2.5000 1400.00 198 27.0 143 20.0 50.0 90.0 140  3 1992 WINTER 39 126.22 11.250 600.00 133 21.2 105 11.3 40.0 85.0 150  3 1993 FALL 38 104.87 20.000 300.00 65.3 10.6 62 20.0 50.0 90.0 150  3 1993 SPRING 48 86.35 20.000 340.00 67.7 9.77 78 20.0 45.0 70.0 100  3 1993 SUMMER 46 98.48 10.000 300.00 75.6 11.1 77 20.0 40.0 80.0 130  3 1993 WINTER 40 103.63 10.000 370.00 86.0 13.6 83 20.0 40.0 80.0 133  3 1994 FALL 61 103.93 5.0000 400.00 85.3 10.9 82 10.0 50.0 90.0 135  3 1994 SPRING 59 86.17 2.5000 460.00 85.8 11.2 100 10.0 30.0 60.0 100  3 1994 SUMMER 70 105.68 2.5000 670.00 106 12.7 100 20.0 40.0 80.0 130  3 1994 WINTER 46 101.09 .00000 240.00 60.8 8.97 60 10.0 40.0 103 135  3 1995 FALL 48 86.82 2.5000 210.00 56.6 8.17 65 10.0 45.0 72.5 118  3 1995 SPRING 46 83.70 .00000 360.00 81.4 12.0 97 0.00 40.0 60.0 100  3 1995 SUMMER 51 95.93 .00000 380.00 85.7 12.0 89 5.00 40.0 80.0 130  3 1995 WINTER 43 97.79 .00000 300.00 63.0 9.61 64 5.00 55.0 90.0 130  3 1996 FALL 48 103.49 .00000 760.00 115 16.7 111 20.0 42.5 75.0 125  3 1996 SPRING 62 118.71 .00000 790.00 142 18.1 120 30.0 50.0 77.5 125  3 1996 SUMMER 47 75.32 10.000 200.00 51.3 7.49 68 15.0 40.0 65.0 100  3 1996 WINTER 43 116.16 5.0000 265.00 65.0 9.91 56 25.0 50.0 120 170  3 1997 FALL 41 76.71 5.0000 195.00 50.4 7.88 66 10.0 40.0 60.0 110  3 1997 SPRING 46 75.00 5.0000 165.00 43.4 6.39 58 20.0 40.0 72.5 110  3 1997 SUMMER 45 82.22 5.0000 220.00 57.6 8.59 70 15.0 40.0 60.0 120  3 1997 WINTER 42 93.48 6.2500 220.00 49.9 7.70 53 25.0 60.0 95.0 120 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 FALL 1 16.00 16.000 16.00 . . . 16.0 16.0 16.0 16.0  3 1998 SPRING 13 79.08 5.0000 120.00 35.5 9.86 45 5.00 70.0 80.0 110  3 1998 SUMMER 13 77.04 5.0000 150.00 57.4 15.9 75 5.00 17.0 75.0 140  3 1998 WINTER 32 92.50 10.000 195.00 45.9 8.11 50 20.0 57.5 100 130  7 1990 FALL 12 131.25 50.000 250.00 52.5 15.2 40 50.0 103 123 155  7 1990 SPRING 12 128.75 30.000 265.00 60.9 17.6 47 30.0 100 115 140  7 1990 SUMMER 12 145.83 30.000 340.00 83.5 24.1 57 30.0 105 120 145  7 1990 WINTER 14 158.21 50.000 290.00 66.9 17.9 42 50.0 120 140 205  7 1991 FALL 13 142.31 .00000 370.00 102 28.2 71 0.00 110 140 150  7 1991 SPRING 12 170.83 40.000 270.00 62.3 18.0 36 40.0 150 160 195  7 1991 SUMMER 12 185.83 40.000 480.00 108 31.2 58 40.0 150 160 175  7 1991 WINTER 14 190.71 20.000 470.00 120 32.2 63 20.0 140 145 230  7 1992 FALL 12 131.25 10.000 250.00 56.3 16.2 43 10.0 110 135 148  7 1992 SPRING 16 578.36 3.7500 5050.00 1280 320 221 3.75 120 155 198  7 1992 SUMMER 16 368.28 20.000 1900.00 551 138 150 20.0 125 143 318  7 1992 WINTER 15 716.33 50.000 8850.00 2251 581 314 50.0 120 140 160  7 1993 FALL 29 192.20 3.7500 1200.00 257 47.8 134 30.0 50.0 80.0 280  7 1993 SPRING 32 247.19 20.000 890.00 215 38.1 87 50.0 105 180 305  7 1993 SUMMER 32 193.44 15.000 750.00 188 33.3 97 50.0 75.0 100 265  7 1993 WINTER 28 527.50 40.000 8350.00 1538 291 292 60.0 150 185 360  7 1994 FALL 31 207.54 3.7500 1700.00 335 60.2 162 30.0 70.0 100 180  7 1994 SPRING 29 245.52 20.000 2350.00 430 79.8 175 55.0 75.0 120 235  7 1994 SUMMER 29 263.28 30.000 1100.00 267 49.5 101 30.0 80.0 150 340  7 1994 WINTER 27 341.85 35.000 5800.00 1095 211 320 35.0 65.0 90.0 205  7 1995 FALL 11 103.64 70.000 180.00 39.6 11.9 38 70.0 80.0 80.0 130  7 1995 SPRING 15 207.00 40.000 1490.00 360 93.0 174 40.0 70.0 100 180  7 1995 SUMMER 11 106.36 50.000 230.00 56.4 17.0 53 50.0 60.0 80.0 130  7 1995 WINTER 15 190.67 70.000 510.00 114 29.5 60 70.0 110 160 270  7 1996 FALL 9 68.33 2.5000 210.00 72.8 24.3 107 2.50 11.3 40.0 110  7 1996 SPRING 9 76.67 20.000 190.00 62.8 20.9 82 20.0 30.0 70.0 80.0  7 1996 SUMMER 9 82.22 20.000 230.00 86.3 28.8 105 20.0 20.0 30.0 150  7 1996 WINTER 9 104.72 2.5000 320.00 120 40.2 115 2.50 30.0 50.0 140 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Turbidity_FTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 29 6.02 .90000 34.00 5.71 1.06 95 2.50 4.00 5.00 7.00  3 1990 SPRING 26 6.23 1.5000 17.00 4.04 0.79 65 2.00 4.00 5.00 7.50  3 1990 SUMMER 29 4.84 1.0000 31.00 5.89 1.09 122 1.00 2.50 3.00 5.00  3 1990 WINTER 31 20.63 1.0000 150.00 35.0 6.28 170 2.00 6.50 9.00 19.0  3 1991 FALL 48 5.31 .25000 26.00 4.89 0.71 92 0.80 2.48 4.00 6.18  3 1991 SPRING 35 13.07 .25000 52.00 10.4 1.76 80 0.95 5.35 12.0 18.0  3 1991 SUMMER 35 4.07 .62500 16.00 3.23 0.55 79 0.80 2.00 3.00 5.00  3 1991 WINTER 32 22.39 2.0000 60.50 16.6 2.93 74 2.70 10.3 19.5 26.8  3 1992 FALL 51 4.73 .25000 22.00 4.58 0.64 97 0.30 2.00 3.50 6.10  3 1992 SPRING 48 7.31 .25000 25.00 5.24 0.76 72 0.60 3.38 6.83 9.98  3 1992 SUMMER 47 4.76 .35000 27.00 4.91 0.72 103 1.00 2.00 3.00 5.40  3 1992 WINTER 35 19.87 .25000 100.00 22.8 3.86 115 1.50 6.00 12.0 23.0  3 1993 FALL 33 4.22 1.0000 16.00 3.45 0.60 82 1.00 2.00 3.00 5.50  3 1993 SPRING 44 7.58 .60000 18.00 4.29 0.65 57 2.00 4.08 7.45 9.75  3 1993 SUMMER 40 4.71 .25000 24.00 4.10 0.65 87 1.00 2.00 3.73 6.05  3 1993 WINTER 36 11.73 1.0000 39.00 8.90 1.48 76 1.30 5.00 11.3 16.5  3 1994 FALL 42 5.56 .25000 24.50 5.67 0.88 102 1.00 2.00 4.00 6.30  3 1994 SPRING 37 5.79 1.0000 40.00 6.89 1.13 119 1.00 2.00 4.00 6.00  3 1994 SUMMER 44 5.32 .62500 27.00 4.56 0.69 86 1.00 3.00 4.00 6.45  3 1994 WINTER 34 14.49 .40000 37.50 9.93 1.70 69 1.50 6.00 13.4 20.0  3 1995 FALL 39 13.88 2.0000 178.00 28.7 4.60 207 2.00 4.00 6.00 10.5  3 1995 SPRING 34 6.94 1.0000 30.00 5.20 0.89 75 2.00 4.00 6.00 8.00  3 1995 SUMMER 36 4.69 .25000 18.00 3.67 0.61 78 0.25 3.00 4.00 5.75  3 1995 WINTER 35 25.11 3.0000 104.00 19.9 3.36 79 3.50 11.0 21.0 32.0  3 1996 FALL 40 8.90 1.0000 43.00 7.31 1.16 82 2.85 4.00 6.45 11.5  3 1996 SPRING 37 15.63 1.0000 71.00 11.4 1.88 73 5.00 9.40 13.7 18.0  3 1996 SUMMER 40 7.35 1.0000 38.00 8.30 1.31 113 1.00 2.85 5.25 7.40  3 1996 WINTER 34 39.03 3.0000 145.00 29.1 4.99 75 6.00 18.0 35.5 49.5  3 1997 FALL 36 7.92 1.0000 21.00 4.72 0.79 60 1.00 5.18 7.28 10.0  3 1997 SPRING 38 10.82 2.0000 52.00 8.30 1.35 77 2.00 6.75 9.00 13.5  3 1997 SUMMER 37 6.67 1.0000 22.50 4.79 0.79 72 2.00 4.00 5.80 7.50  3 1997 WINTER 36 22.33 1.5000 77.50 13.2 2.21 59 5.00 15.0 22.8 26.0 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Turbidity_FTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1998 SPRING 12 10.38 4.0000 21.00 4.49 1.30 43 4.00 7.63 9.25 12.4  3 1998 SUMMER 6 9.54 3.8500 21.50 6.13 2.50 64 3.85 6.90 8.13 8.75  3 1998 WINTER 16 21.67 2.0000 50.00 10.5 2.63 49 2.00 17.0 20.3 26.5  7 1990 FALL 12 7.04 2.0000 14.00 3.17 0.91 45 2.00 5.00 6.25 8.50  7 1990 SPRING 12 9.23 2.3000 18.00 4.83 1.39 52 2.30 6.00 8.50 12.0  7 1990 SUMMER 12 11.88 2.6000 26.00 6.91 1.99 58 2.60 8.00 10.5 14.3  7 1990 WINTER 12 12.02 5.0000 36.00 8.66 2.50 72 5.00 6.00 9.80 15.0  7 1991 FALL 12 8.96 1.3000 16.00 4.10 1.18 46 1.30 6.50 8.50 11.3  7 1991 SPRING 12 14.83 5.5000 22.50 5.47 1.58 37 5.50 10.5 14.5 19.8  7 1991 SUMMER 12 12.73 3.8000 29.00 7.05 2.03 55 3.80 7.50 10.8 16.5  7 1991 WINTER 12 6.83 3.5000 16.00 3.37 0.97 49 3.50 5.00 6.00 7.50  7 1992 FALL 11 7.91 5.0000 14.00 2.98 0.90 38 5.00 5.00 8.00 10.0  7 1992 SPRING 12 12.53 4.9000 18.50 4.48 1.29 36 4.90 9.25 11.3 16.8  7 1992 SUMMER 13 10.74 5.0000 19.00 4.46 1.24 42 5.00 7.00 10.5 12.0  7 1992 WINTER 12 7.10 3.0000 10.00 2.26 0.65 32 3.00 5.50 7.50 9.00  7 1993 FALL 12 11.21 5.0000 32.00 8.02 2.32 72 5.00 6.00 8.75 11.5  7 1993 SPRING 12 11.78 3.9000 18.00 4.27 1.23 36 3.90 8.00 12.8 15.0  7 1993 SUMMER 12 12.37 .60000 28.50 7.47 2.16 60 0.60 7.75 11.5 14.8  7 1993 WINTER 12 29.33 2.0000 54.00 16.4 4.73 56 2.00 18.0 26.0 43.5  7 1994 FALL 12 7.31 1.2000 19.50 5.07 1.46 69 1.20 4.25 6.25 7.50  7 1994 SPRING 12 9.11 .80000 20.00 5.36 1.55 59 0.80 4.75 9.25 12.3  7 1994 SUMMER 12 11.65 .30000 28.50 8.05 2.32 69 0.30 6.50 9.00 16.5  7 1994 WINTER 11 7.36 5.0000 10.00 1.78 0.54 24 5.00 5.00 7.50 9.00  7 1995 FALL 11 7.77 5.0000 14.50 2.98 0.90 38 5.00 5.00 7.00 10.0  7 1995 SPRING 11 15.59 7.0000 26.50 5.88 1.77 38 7.00 10.0 15.0 21.0  7 1995 SUMMER 11 12.05 7.0000 20.00 4.37 1.32 36 7.00 9.00 11.0 15.5  7 1995 WINTER 11 29.05 11.500 62.00 19.3 5.82 66 11.5 13.5 20.5 53.5 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1995 to 1995 Turbidity_JCU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  7 1995 SPRING 1 12.00 12.000 12.00 . . . 12.0 12.0 12.0 12.0  7 1995 WINTER 1 78.00 78.000 78.00 . . . 78.0 78.0 78.0 78.0 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Turbidity_NTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1990 FALL 1 1.50 1.5000 1.50 . . . 1.50 1.50 1.50 1.50  3 1990 SPRING 1 1.10 1.1000 1.10 . . . 1.10 1.10 1.10 1.10  3 1990 SUMMER 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25  3 1990 WINTER 1 2.10 2.1000 2.10 . . . 2.10 2.10 2.10 2.10  3 1991 FALL 4 4.94 1.0000 11.75 4.87 2.44 99 1.00 1.45 3.50 8.43  3 1991 SPRING 1 1.50 1.5000 1.50 . . . 1.50 1.50 1.50 1.50  3 1991 SUMMER 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20  3 1991 WINTER 4 3.00 1.0000 6.00 2.45 1.22 82 1.00 1.00 2.50 5.00  3 1992 FALL 3 7.10 1.3000 17.00 8.62 4.97 121 1.30 1.30 3.00 17.0  3 1992 SPRING 4 3.71 .60000 9.90 4.29 2.15 116 0.60 0.83 2.18 6.60  3 1992 SUMMER 4 5.44 .60000 16.00 7.16 3.58 132 0.60 1.03 2.58 9.85  3 1992 WINTER 4 7.20 2.3000 13.00 5.43 2.72 75 2.30 2.58 6.75 11.8  3 1994 FALL 4 4.67 1.5125 7.50 2.93 1.47 63 1.51 2.18 4.83 7.15  3 1994 WINTER 4 5.38 1.2000 12.00 4.63 2.32 86 1.20 2.60 4.15 8.15  3 1995 FALL 4 9.43 2.5000 16.00 5.52 2.76 59 2.50 5.85 9.60 13.0  3 1995 SPRING 4 2.10 .60000 4.70 1.83 0.91 87 0.60 0.85 1.55 3.35  3 1995 SUMMER 4 1.68 .60000 4.40 1.82 0.91 109 0.60 0.65 0.85 2.70  3 1995 WINTER 4 2.63 .70000 6.30 2.51 1.25 96 0.70 1.10 1.75 4.15  3 1996 FALL 7 20.82 .60000 44.15 15.7 5.94 75 0.60 5.50 18.2 36.7  3 1996 SPRING 1 2.10 2.1000 2.10 . . . 2.10 2.10 2.10 2.10  3 1996 SUMMER 1 1.40 1.4000 1.40 . . . 1.40 1.40 1.40 1.40  3 1996 WINTER 8 21.75 2.5500 41.60 14.1 4.99 65 2.55 12.7 16.8 35.4  3 1997 FALL 11 17.54 10.000 34.50 7.07 2.13 40 10.0 12.0 16.9 18.5  3 1997 SPRING 10 17.59 8.4000 29.75 7.13 2.25 41 8.40 11.8 14.8 24.4  3 1997 SUMMER 7 12.42 6.0300 23.15 5.52 2.09 44 6.03 8.60 12.7 14.2  3 1997 WINTER 19 18.72 2.5000 89.20 19.5 4.48 104 2.50 7.00 13.0 23.6  3 1998 WINTER 14 28.68 4.5000 69.00 19.9 5.32 69 4.50 7.50 34.3 40.0  7 1991 FALL 2 4.30 2.7000 5.90 2.26 1.60 53 2.70 2.70 4.30 5.90  7 1992 WINTER 1 7.70 7.7000 7.70 . . . 7.70 7.70 7.70 7.70 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1992 to 1998 pH_S_U subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  3 1993 FALL 5 7.49 7.4000 7.55 0.05 0.02 1 7.40 7.50 7.50 7.50  3 1993 SPRING 5 7.12 6.9000 7.40 0.19 0.09 3 6.90 7.00 7.10 7.20  3 1993 SUMMER 5 7.50 7.3500 7.70 0.15 0.07 2 7.35 7.40 7.45 7.60  3 1993 WINTER 5 7.28 7.1500 7.40 0.09 0.04 1 7.15 7.25 7.30 7.30  3 1994 FALL 5 6.95 6.8000 7.15 0.13 0.06 2 6.80 6.90 6.90 7.00  3 1994 SPRING 5 7.27 7.1500 7.50 0.14 0.06 2 7.15 7.20 7.20 7.30  3 1994 SUMMER 5 7.27 7.2000 7.35 0.06 0.03 1 7.20 7.25 7.25 7.30  3 1994 WINTER 5 7.17 6.9000 7.35 0.16 0.07 2 6.90 7.20 7.20 7.20  3 1995 FALL 2 7.20 7.2000 7.20 0.00 0.00 0 7.20 7.20 7.20 7.20  3 1995 SPRING 5 7.26 7.0000 7.70 0.26 0.12 4 7.00 7.20 7.20 7.20  3 1995 SUMMER 5 7.35 7.1000 7.50 0.15 0.07 2 7.10 7.35 7.40 7.40  3 1995 WINTER 5 7.16 7.0000 7.20 0.09 0.04 1 7.00 7.20 7.20 7.20  3 1996 FALL 9 7.09 6.8000 7.40 0.20 0.07 3 6.80 6.94 7.11 7.21  3 1996 SPRING 7 7.39 6.9500 7.67 0.25 0.09 3 6.95 7.19 7.42 7.57  3 1996 SUMMER 8 7.40 6.8000 7.69 0.29 0.10 4 6.80 7.30 7.50 7.58  3 1996 WINTER 7 7.16 6.6000 7.50 0.30 0.11 4 6.60 7.01 7.25 7.40  3 1997 FALL 7 7.69 7.5000 8.20 0.27 0.10 3 7.50 7.50 7.50 7.80  3 1997 SPRING 7 7.41 7.0000 7.75 0.28 0.11 4 7.00 7.20 7.55 7.60  3 1997 SUMMER 7 7.56 7.4000 7.80 0.13 0.05 2 7.40 7.50 7.50 7.60  3 1997 WINTER 7 7.21 6.7000 7.50 0.32 0.12 4 6.70 6.90 7.35 7.50  3 1998 FALL 1 8.05 8.0500 8.05 . . . 8.05 8.05 8.05 8.05  3 1998 SPRING 1 7.50 7.5000 7.50 . . . 7.50 7.50 7.50 7.50  3 1998 SUMMER 6 7.96 7.5000 8.50 0.41 0.17 5 7.50 7.65 7.85 8.40  7 1992 SPRING 3 8.12 7.8000 8.35 0.28 0.16 4 7.80 7.80 8.20 8.35  7 1992 SUMMER 3 8.47 7.8000 9.40 0.83 0.48 10 7.80 7.80 8.20 9.40  7 1992 WINTER 3 7.68 7.6000 7.75 0.08 0.04 1 7.60 7.60 7.70 7.75  7 1993 FALL 9 7.61 7.3000 7.95 0.22 0.07 3 7.30 7.40 7.65 7.80  7 1993 SPRING 9 7.68 7.2000 8.30 0.34 0.11 4 7.20 7.50 7.60 7.80  7 1993 SUMMER 9 7.81 7.4000 8.10 0.23 0.08 3 7.40 7.60 7.85 8.00  7 1993 WINTER 9 7.62 7.1000 8.00 0.28 0.09 4 7.10 7.50 7.60 7.80  7 1994 FALL 9 7.77 7.4000 8.10 0.22 0.07 3 7.40 7.60 7.85 7.90  7 1994 SPRING 9 7.74 7.3500 8.10 0.27 0.09 4 7.35 7.50 7.80 7.95 
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: I  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1992 to 1998 pH_S_U subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  7 1994 SUMMER 9 7.84 7.4000 8.10 0.22 0.07 3 7.40 7.80 7.90 8.00  7 1994 WINTER 9 7.64 7.2000 8.15 0.34 0.11 4 7.20 7.30 7.70 7.85  7 1995 SPRING 3 7.40 7.0000 8.00 0.53 0.31 7 7.00 7.00 7.20 8.00  7 1995 WINTER 3 7.13 6.3000 7.70 0.74 0.43 10 6.30 6.30 7.40 7.70 
APPENDIX C

Quality Control/Quality Assurance Rules

Appendix C—Quality Control/Quality Assurance Rules

Continued Support for the Compilation and

Analysis of National Nutrient Data

9 Nutrient Ecoregion/Waterbody Type Summary

Chapters

Prepared for:

Steve Potts

Environmental Protection Agency

OW/OST/HECD

Prepared by:

INDUS Corporation

1953 Gallows Road

Vienna, Virginia 22182

Contract Number:68-C-99-226

Task Number:07

Subtask Number:4

August 27, 2001

9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-iii

CONTENTS

1.0 BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

1.2 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.0 QA/QC PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.1 National Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

2.2 State Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

2.3 Laboratory Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

2.4 Waterbody Name and Class Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

2.5 Ecoregion Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

3.0 STATISTICAL ANALYSIS REPORTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

3.1 Data Source Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

3.2 Remark Code Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

3.3 Median of Each Waterbody . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

3.4 Descriptive Statistic Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

3.5 Regression Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

4.0 TIME PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

5.1 Lakes and Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.1 Aggregate Nutrient Ecoregion 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.2 Aggregate Nutrient Ecoregion 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.3 Aggregate Nutrient Ecoregion 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

5.1.4 Aggregate Nutrient Ecoregion 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

5.2 Rivers and Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

5.2.1 Aggregate Nutrient Ecoregion 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

5.2.2 Aggregate Nutrient Ecoregion 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-12

5.2.3 Aggregate Nutrient Ecoregion 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-13

5.2.4 Aggregate Nutrient Ecoregion 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-13

5.2.5 Aggregate Nutrient Ecoregion 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-14

APPENDIX A Process Used to QA/QA the Legacy STORET Nutrient Data Set . . . . . . C-16

APPENDIX B Process for Adding Aggregate Nutrient Ecoregions and Level III 

Ecoregions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-22

APPENDIX C Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-1

1.0 BACKGROUND

The Nutrient Criteria Program initiated the development of a national Nutrient Criteria Database

application that is used to store and analyze nutrient data. The ultimate use of these data is to

derive ecoregion specific nutrient criteria. EPA converted STOrage and RETrieval (STORET)

legacy data, National Stream Quality Accounting Network (NASQAN) data, National WaterQuality Assessment (NAWQA) data, and other relevant nutrient data from universities and

States/Tribes into the database. The data imported into the Nutrient Criteria Database are used

to develop national nutrient criteria recommendations. 

1.1 Purpose

The purpose of this deliverable is to provide EPA with information regarding the database used

to create the statistical reports which will be used to derive ecoregion-specific nutrient criteria

for Level III ecoregions. There are fourteen aggregate nutrient ecoregions. Each aggregate

nutrient ecoregion is divided into smaller ecoregions (subecoregions) referred to as Level III

ecoregions. EPA will determine criteria for the waterbody types and Level III ecoregions within

the following aggregate nutrient ecoregions:

• Lakes and Reservoirs

- Aggregate Nutrient ecoregions: 3, 4, 5, and 14

• Rivers and Streams 

- Aggregate Nutrient ecoregions: 1, 4, 5, 8, and 10

1.2 References

This section lists documents that contain baselines, standards, guidelines, policies, and

references that apply to the data analysis. Listed editions were valid at the time of publication. 

All documents are subject to revision, but these specific editions govern the concepts described

in this document. 

Nutrient Criteria Technical Guidance Document: Lakes and Reservoirs (Draft). EPA, Office of

Water, EPA 822-D-99-001, April 1999.

Nutrient Criteria Technical Guidance Manual: Rivers and Streams (Draft). EPA, Office of

Water, EPA 822-D-99-003, September 1999.

Guidance for Data Quality Assessment: Practical Methods for Data Analysis. EPA, Office of

Research and Development, EPA QA/G-9, January 1998.

2.0 QA/QC PROCEDURES

In order to develop nutrient criteria, EPA needed to obtain nutrient data from the states. EPA

requested nutrient data from the states and forwarded the data sets to INDUS via e-mail and/or

US mail. In addition, EPA tasked INDUS to convert data from three national data sets. EPA
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-2 Appendix C—Quality Control/Quality Assurance Rules

provided INDUS with a Legacy STORET extraction to convert into the database. The United

States Geologic Survey (USGS) sent INDUS a CD-ROM with NASQAN data to convert. 

INDUS downloaded NAWQA files from the USGS Web site to convert the data. In total,

INDUS converted and imported the following national and state data sets into the Nutrient

Criteria Database:

• Legacy STORET

• NAWQA

• NASQAN

• EPA Region 1

• EPA Region 2 - Lake Champlain Monitoring Project

• EPA Region 2 - NYSDEC Finger Lakes Monitoring Program

• EPA Region 2 - NY Citizens Lake Assessment Program

• EPA Region 2 - Lake Classification and Inventory Survey

• EPA Region 2 - NYCDEP (1990-1998)

• EPA Region 2 - NYCDEP (Storm Event data)

• EPA Region 2 - New Jersey Nutrient Data ( Tidal Waters)

• EPA Region 5

• EPA Region 3 

• EPA Region 3 - Nitrite Data

• EPA Region 3 - Choptank River files

• EPA Region 4 - Tennessee Valley Authority

• EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 

• EPA Region 7 - REMAP

• EPA Region 2 - Delaware River Basin Commission (1990-1998)

• EPA Region 3 - PA Lake Data

• EPA Region 3 - University of Delaware

• EPA Region 10 

• University of Auburn

• EPA Region 8 - MT and WY

• EPA Region 9

• Suffolk County

• NYCDEC

• NY Lakes Morphometry

• EPA Region 8 - South Dakota

• EPA Region 8 - Colorado Reservoir

• EPA Region 4

• EPA Region 10 - Lake Data

• EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

• EPA Region 8 - North Dakota

• EPA Region 8 - Eagle River

• EPA Region 8 - Utah

• Florida
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-3

As part of the conversion process, INDUS performed a number of Quality Assurance/Quality

Control (QA/QC) steps to ensure that the data were properly converted into the Nutrient Criteria

Database. Sections 2.1 and 2.2 explain the steps performed by INDUS to convert the data.

2.1 National Data Sets

INDUS converted three national data sets into the Nutrient Criteria Database: Legacy STORET

data, NASQAN data, and NAWQA data. A previous EPA contractor performed the extraction

of Legacy STORET data and documented the QA/QC procedures used on the data. This

documentation is included in Appendix A. INDUS performed minimal QA/QC on the Legacy

STORET data set because the previous contractor completed the steps outlined in Appendix A. 

INDUS and EPA also agreed to convert the NAWQA and NASQAN data sets with minimal

QA/QC on the assumption that the source agency, the USGS, QA/QC’d the data. 

For each of the three national data sets, INDUS ran queries to determine if 1) samples existed

without results and 2) if stations existed without samples. Per Task Order Project Officer

(TOPO) direction, these records were deleted from the system. For analysis purposes, EPA

determined that there was no need to keep station records with no samples and sample records

with no results. INDUS also confirmed that each data set contained no duplicate records. 

In addition, INDUS deleted all composite results from the Legacy STORET data. Per TOPO

direction, it was decided that composite sample results would not be used in the statistical

analysis.

2.2 State Data

Each state data set was delivered in a unique format. Many of the data sets were delivered to

INDUS without corresponding documentation. INDUS analyzed each state data set in order to

determine which parameters should be converted for analysis. INDUS obtained a master

parameter table from EPA and converted the parameters in the state data sets according to those

that were present in the EPA parameter table. INDUS converted all of the data elements in the

state data sets that mapped directly to the Nutrient Criteria Database; data elements that did not

map to the Nutrient Criteria Database were not converted. In some cases, state data elements

that did not directly map into the Oracle database were inserted into a comment field within the

database. Also, INDUS maintained an internal record of which state data elements were inserted

into the comment field. 

As part of the data clean-up efforts, INDUS determined whether or not there were any duplicate

records in the state data sets and deleted the duplicate records. INDUS checked the waterbody,

station, and sample entities for duplicate records. However, if there was not enough information

provided to determine duplicates such as sampling date, there was no way for INDUS to locate

duplicate records. In addition, INDUS deleted station records with no samples and sample

records with no results. INDUS also deleted waterbody records that were not associated with a

station. In each case, INDUS maintained an internal record of how many records were deleted. 
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-4 Appendix C—Quality Control/Quality Assurance Rules

If INDUS encountered referential integrity errors, such as samples that referred to stations that

did not exist, or if INDUS was unsure of whether a record was a duplicate, INDUS contacted the

agency directly via e-mail or phone to resolve any issues that arose. INDUS saved an electronic

copy of each e-mail correspondence with the states to ensure that a record of the decision was

maintained. 

Finally, INDUS examined the remark codes of each result record in the state data sets. INDUS

mapped the remark codes to the STORET remark codes listed in Table 2 of Appendix A. If any

of the state result records were associated with remark codes marked as “Delete” in Table 2 of

Appendix A, the result records were not converted into the database.

2.3 Laboratory Methods 

Many of the state data sets did not contain laboratory method information. In addition,

laboratory method information was not available for the three national data sets. In order to

determine missing laboratory method information, EPA tasked another contractor to contact the

data owners to obtain the laboratory method. In some cases, the data owners responded and the

laboratory methods were added to the database. In other cases, the methods are unknown. 

2.4 Waterbody Name and Class Information

A large percentage of the data did not have waterbody-specific information. The only waterbody

information contained in the three national data sets was the waterbody name, which was

embedded in the station ‘location description’ field. Most of the state data sets contained

waterbody name information; however, much of the data were duplicated throughout the data

sets. Therefore, the waterbody information was cleaned manually. For the three national data

sets, the ‘location description’ field was extracted from the station table and moved to a

temporary table. The ‘location description’ field was sorted alphabetically. Unique waterbodies

were grouped together based on name similarity and whether or not the waterbodies fell within

the same county, state, and waterbody type. Finally, the ‘location description’ field was edited

to include only waterbody name information, not descriptive information. For example, 110

MILE CREEK AT POMONA DAM OUTFLOW, KS PO-2 was edited to 110 MILE CREEK. 

Also, if 100 MILE CREEK was listed ten times in New York, but in four different counties, four

100 MILE CREEK waterbody records were created.

Similar steps were taken to eliminate duplicate waterbody records in the state data sets. If a

number of records had similar waterbody names and fell within the same state, county, and

waterbody type, the records were grouped to create a unique waterbody record.

Most of the waterbody data did not contain depth, surface area, and volume measurements. EPA

needed this information to classify waterbody types. EPA attempted to obtain waterbody class

information from the states. EPA sent waterbody files to the regional coordinators and requested

that certain class information be completed by each state. The state response was poor;

therefore, EPA was not able to perform statistical analysis for the waterbody types by class.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-5

2.5 Ecoregion Data

Aggregate nutrient ecoregions and Level III ecoregions were added to the database using the

station latitude and longitude coordinates, the county centroid, or HUC (Hydrological Unit

Code) centroid. If a station was lacking latitude and longitude coordinates and county

information, the data were not included in the statistical analysis. Appendix B lists the steps

taken to add the two ecoregion types (aggregate and Level III) to the Nutrient Criteria Database. 

The ecoregion names were pulled from aggregate nutrient ecoregion and Level III ecoregion

Geographical Information System (GIS) coverages. In summary, the station latitude and

longitude coordinates were used to determine the ecoregion under the following circumstances:

• The latitude and longitude coordinates fell within the county/state listed in the station table.

• The county data were missing.

The county centroid was used to determine the ecoregions under the following circumstances:

• The latitude and longitude coordinates were missing, but the state/county information was

available.

• The latitude and longitude coordinates fell outside the county/state/HUC listed in the station

table. The county information was assumed to be correct; therefore, the county centroid was

used.

The HUC centroid was used to determine the ecoregions under the following circumstances:

• The latitude and longitude coordinates and county were missing, but the HUC information

was available.

If the latitude and longitude coordinates fell outside the continental US county coverage file 

(i.e., the point fell in the ocean or Mexico/Canada), the nearest ecoregion was assigned to the

station.

3.0 STATISTICAL ANALYSIS REPORTS

Aggregate nutrient ecoregion tables were created by extracting all observations for a specific

aggregate nutrient ecoregion from the Nutrient Criteria Database. Then, the data were reduced

to create tables containing only the yearly median values. To create these tables, the median

value for each waterbody was calculated using all observations for each waterbody by Level III

ecoregion, state, county, year, and season. Tables of decade median values were created from

the yearly median tables by calculating the median for each waterbody by Level III ecoregion,

state, county, decade and season.

The Data Source and the Remark Code reports were created using all observations (all reported

values). All the other reports were created from either the yearly median tables or the decade

median tables. In other words, the descriptive statistics and regressions were run using the

median values for each waterbody and not the individual reported values.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-6 Appendix C—Quality Control/Quality Assurance Rules

Statistical analyses were performed under the assumption that this data set is a random sample. 

If this assumption cannot be verified, the observations may or may not be valid. Values below

the 1st and 99th percentile were removed from the Legacy STORET database prior to the creation

of the national database. Also, data were treated according to the Legacy STORET remark codes

in Appendix A. 

The following contains a list of each report and the purpose for creating each report: 

• Data Source—Created to provide a count of the amount of data and to identify the source(s).

• Remark Codes—Created to provide a description of the data. 

• Median of Each Waterbody by Year—This was an intermediate step performed to obtain a

median value for each waterbody to be used in the yearly descriptive statistics reports and the

regression models. 

• Median of Each Waterbody by Decade—This was an intermediate step performed to obtain a

median value for each waterbody to be used in the decade descriptive statistics.

• Descriptive Statistics—Created to provide EPA with the desired statistics for setting criteria

levels.

• Regression Models—Created to examine the relationships between biological and nutrient

variables. 

Note: Separate reports were created for each season.

3.1 Data Source Reports

Data source reports were presented in the following formats:

• The number and percentage of data from each data source were summarized in tables for

each aggregate nutrient ecoregion by season and waterbody type.

• The number and percentage of data from each data source were summarized in tables for

each aggregate nutrient ecoregion for all seasons and waterbody type.

• The number and percentage of data from each data source were summarized in tables for

each Level III ecoregion by season and waterbody type.

The ‘Frequency’ represents the number of data values from a specific data source for each

parameter by data source. The ‘Row Pct’ represents the percentage of data from a specific data

source for each parameter. 

3.2 Remark Code Reports

Remark code reports were presented in the following formats:

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by decade and season.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-7

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by year and season.

The ‘Frequency’ represents the number of data values corresponding to the remark code in the

column. The ‘Row Pct’ represents the percentage of data that was associated with the remark

code in that row.

In the database, remark codes that were entered by the states were mapped to Legacy STORET

remark codes. Prior to the analysis, the data were treated according to these remark codes. For

example, if the remark code was ‘K,’ then the reported value was divided by two. Appendix A

contains a complete list of Legacy STORET remark codes.

Note: For the reports, a remark code of ‘Z’ indicates that no remark codes were recorded. It does

not correspond to Legacy STORET code ‘Z.’

3.3 Median of Each Waterbody

To reduce the data and to ensure heavily sampled waterbodies or years were not over represented

in the analysis, median value tables (described above) were created. The yearly median tables

and decade median tables were delivered to the EPA in electronic format as csv (comma

separated value or comma delimited) files.

3.4 Descriptive Statistic Reports

The number of waterbodies, median, mean, minimum, maximum, 5th, 25th , 75th , 95th percentiles,

standard deviation, standard error, and coefficient of variation were calculated. The tables

(described above) containing the decade median values for each waterbody for each parameter

were used to create descriptive statistics reports for:

• Level III ecoregions by decade and season

• Aggregate nutrient ecoregions by decade and season

In addition, the tables containing the yearly median values for each waterbody for each

parameter were used to create descriptive statistics reports for:

• Level III ecoregions by year and season

3.5 Regression Models

Simple linear regressions using the least squares method were performed to examine the

relationships between biological and nutrient variables in lakes and reservoirs, and rivers and

streams. Regressions were performed using the yearly median tables. Chlorophyll(s) in

micrograms per liter (ug/L), Secchi in meters (m), Dissolved Oxygen in milligrams per liter

(mg/L), Turbidity, and pH were the biological variables in these models. Secchi data were used

in the lake and reservoir models, and Turbidity data were used in the river and stream models. 
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-8 Appendix C—Quality Control/Quality Assurance Rules

The nutrient variables in these models include: Total Phosphorus in ug/L, Total Nitrogen in

mg/L, Total Kjeldahl Nitrogen in mg/L, and Nitrate and Nitrite in mg/L. 

4.0 TIME PERIOD

Data collected from January 1990 to December 2000 were used in the statistical analysis reports. 

To capture seasonal differences, the data were classified as follows:

• Aggregate nutrient ecoregions: 6, 7, and 8

S Spring: April to May 

S Summer: June to August 

S Fall: September to October

S Winter: November to March

• Aggregate nutrient ecoregions: 1, 2, 3, 4, 5, 9, 10, 11, 12, 13, and 14

S Spring: March to May

S Summer: June to August 

S Fall: September to November

S Winter: December to February 

5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS

This section provides information for the nutrient aggregate ecoregions that were analyzed by

waterbody type. Each section lists the data sources for the aggregate nutrient ecoregion

including: 1) the data sources, 2) the parameters included in the analysis, and 3) the Level III

ecoregions within the aggregate nutrient ecoregions.

Note: For analysis purposes, data for the following parameters were grouped together and

reported under Phosphorous, Dissolved Inorganic (DIP):

Phosphorus, Dissolved Inorganic (DIP) 

Phosphorus, Dissolved (DP) 

Phosphorus, Dissolved Reactive (DRP)

Orthophosphate, dissolved, mg/L as P 

Orthophosphate (OPO4_PO4)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-9

5.1 Lakes and Reservoirs

5.1.1 Aggregate Nutrient Ecoregion 3

Data Sources:

Legacy STORET

EPA Region 10

EPA Region 8 - Colorado Reservoir

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

6, 10, 12, 13, 18, 20, 22, 24, 80, 81

5.1.2 Aggregate Nutrient Ecoregion 4

Data Sources:

Legacy STORET

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-10 Appendix C—Quality Control/Quality Assurance Rules

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

26, 28, 30, 31, 43, 44

5.1.3 Aggregate Nutrient Ecoregion 5

Data sources:

Legacy STORET

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

25, 27, 32, 42

5.1.4 Aggregate Nutrient Ecoregion 14

Data sources:

Legacy STORET

Region 2 - NY Citizens Lake Assessment Program

Region 2 - NYCDEP (1990-1998)

EPA Region 1
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-11

Parameters:

CHLB (ug/L)

CHLC (ug/L)

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric, uncorrected (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

59, 63, 84

5.2 Rivers and Streams

5.2.1 Aggregate Nutrient Ecoregion 1

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 10

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Periphyton, spectrophotometric, uncorrected (mg/sqm)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-12 Appendix C—Quality Control/Quality Assurance Rules

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

3, 7

5.2.2 Aggregate Nutrient Ecoregion 4

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

26, 28, 30, 31, 43, 44
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-13

5.2.3 Aggregate Nutrient Ecoregion 5

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P (ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

25, 27, 32, 42

5.2.4 Aggregate Nutrient Ecoregion 8 

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 2 - NYCDEP (1990-1998)

EPA Region 1
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-14 Appendix C—Quality Control/Quality Assurance Rules

EPA Region 3

EPA Region 5

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric, uncorrected (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P (ug/L)

Turbidity (FTU)

Turbidity (NTU)

pH

Level III ecoregions:

49, 50, 58, 62, 82

5.2.5 Aggregate Nutrient Ecoregion 10

Data sources:

Legacy STORET

NASQAN

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-15

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

34, 73
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-16 Appendix C—Quality Control/Quality Assurance Rules

APPENDIX A. Process Used to QA/QC the Legacy STORET Nutrient Data Set

1. STORET water quality parameters and Station and Sample data items were retrieved from

USEPA’s mainframe computer. Table 1 lists all retrieved parameters and data items.

TABLE 1: PARAMETERS AND DATA ITEMS RETRIEVED FROM STORET

Parameters Retrieved

 (STORET Parameter Code) 

Station Data Items Included

(STORET Item Name)

Sample Data Items

Included

(STORET Item Name)

TN - mg/l (600)

TKN - mg/l (625)

Total Ammonia (NH3+NH4) - mg/l (610)

Total NO2+NO3 - mg/l (630)

Total Nitrite - mg/l (615)

Total Nitrate - mg/l (620)

Organic N - mg/L (605)

TP - mg/l (665)

Chlor a - ug/L (spectrophotometric method,

32211) 

Chlor a - ug/L (fluorometric method corrected,

32209)

Chlor a - ug/L (trichromatic method corrected,

32210)

Secchi Transp. - inches (77)

Secchi Transp. - meters (78)

+Turbidity JCUs (70)

+Turbidity FTUs (76)

+Turbidity NTUs field (82078)

+Turbidity NTUs lab (82079)

+DO - mg/L (300)

+Water Temperature (degrees C, 10/degrees F,

11)

Station Type (TYPE)

Agency Code (AGENCY)

Station No. (STATION)

Latitude - std. decimal degrees

(LATSTD)

Longitude - std. decimal degrees 

(LONGSTD)

Station Location (LOCNAME)

County Name (CONAME)

State Name (STNAME)

Ecoregion Name - Level III

(ECONAME)

Ecoregion Code -Level III

(ECOREG)

Station Elevation (ELEV)

Hydrologic Unit Code

(CATUNIT)

RF1 Segment and Mile

(RCHMIL)

RF1ON/OFF tag (ONOFF)

Sample Date (DATE)

Sample Time (TIME)

Sample Depth (DEPTH)

Composite Sample Code 

(SAMPMETH)

+ If data record available at a station included data only for this or other such marked parameters, data record was

deleted from data set. 

The following set of retrieval rules were applied to the retrieval process:

C Data were retrieved for waterbodies specified only as ‘lake’, ‘stream’, ‘reservoir’, or

‘estuary’ under “Station Type” parameter. Any stations specified as ‘well,’ ‘spring,’ or

‘outfall’ were eliminated from the retrieved data set.

• Data were retrieved for station types described as ‘ambient’ (e.g., no pipe or facility

discharge data) under the “Station Type” parameter.

• Data were retrieved that were designated as ‘water’ samples only. This includes ‘bottom’

and ‘vertically integrated’ water samples.

• Data were retrieved that were designated as either ‘grab’ samples and ‘composite’ samples

(mean result only).
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-17

• No limits were specified for sample depths.

• Data were retrieved for all fifty states, Puerto Rico, and the District of Columbia.

• The time period specified for data retrieval was January 1990 to September 1998.

• No data marked as “Retired Data” (i.e., data from a generally unknown source) were

retrieved.

• Data marked as “National Urban Runoff data” (i.e., data associated with sampling conducted

after storm events to assess nonpoint source pollutants) were included in the retrieval. Such

data are part of STORET’s ‘Archived’ data. 

• Intensive survey data (i.e., data collected as part of specific studies) were retrieved.

2. Any values falling below the 1st percentile and any values falling above the 99th

percentile were transformed into ‘missing’ values (i.e., values were effectively removed

from the data set, but were not permanently eliminated).

3. Based on the STORET ‘Remark Code’ associated with each retrieved data point, the

following rules were applied (Table 2): 

TABLE 2: STORET REMARK CODE RULES

STORET Remark Code Keep or Delete Data Point

blank - Data not remarked. Keep

A - Value reported is the mean of two or more determinations. Keep 

B - Results based upon colony counts outside the acceptable ranges. Delete

C -Calculated. Value stored was not measured directly, but was

calculated from other data available.

Keep

D - Field measurement. Keep

E - Extra sample taken in compositing process. Delete

F - In the case of species, F indicates female sex. Delete

G - Value reported is the maximum of two or more determinations. Delete

H - Value based on field kit determination; results may not be accurate. Delete

I - The value reported is less than the practical quantification limit and

greater than or equal to the method detection limit.

Keep, but used one-half the 

reported value as the new value.

J - Estimated. Value shown is not a result of analytical measurement. Delete
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

TABLE 2: STORET REMARK CODE RULES

C-18 Appendix C—Quality Control/Quality Assurance Rules

K - Off-scale low. Actual value not known, but known to be less than

value shown.

Keep, but used one-half the reported 

value as the new value.

L - Off-scale high. Actual value not known, but known to be greater

than value shown.

Keep

M -Presence of material verified, but not quantified. Indicates a positive

detection, at a level too low to permit accurate quantification.

Keep, but used one half the reported

value as the new value. 

N -Presumptive evidence of presence of material. Delete

O -Sample for, but analysis lost. Accompanying value is not meaningful

for analysis.

Delete

P -Too numerous to count. Delete

Q -Sample held beyond normal holding time. Delete

R -Significant rain in the past 48 hours. Delete

S -Laboratory test. Keep

T -Value reported is less than the criteria of detection. Keep, but replaced reported value with

0.

U -Material was analyzed for, but not detected. Value stored is the limit

of detection for the process in use. 

Keep, but replaced reported value with

0.

V -Indicates the analyte was detected in both the sample and associated

method blank.

Delete

W -Value observed is less than the lowest value reportable under remark

“T.”

Keep, but replaced reported value with

0.

X -Value is quasi vertically-integrated sample. No data point with this remark code in 

data set.

Y -Laboratory analysis from unpreserved sample. Data may not be

accurate.

Delete

Z -Too many colonies were present to count. Delete

If a parameter (excluding water temperature) value was less than or equal to zero and no remark code was present,

the value was transformed into a missing value.

Rationale - Parameter concentrations should never be zero without a proper explanation. A method detection limit

should at least be listed
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-19

4. Station records were eliminated from the data set if any of the following descriptors were

present within the “Station Type” parameter:

< MONITR - Source monitoring site, which monitors a known problem or to detect a specific

problem.

< HAZARD - Site of hazardous or toxic wastes or substances.

< ANPOOL - Anchialine pool, underground pools with subsurface connections to watertable

and ocean.

< DOWN - Downstream (i.e., within a potentially polluted area) from a facility which has a

potential to pollute.

< IMPDMT - Impoundment. Includes waste pits, treatment lagoons, and settling and

evaporation ponds.

< STMSWR - Storm water sewer.

< LNDFL - Landfill.

< CMBMI - Combined municipal and industrial facilities.

< CMBSRC - Combined source (intake and outfall).

Rationale - these descriptors potentially indicate a station location that at which an ambient

water sample would not be obtained (i.e., such sampling locations are potentially biased) or the

sample location is not located within one of the designated water body types (i.e, ANPOOL). 

5. Station records were eliminated from data set if the station location did not fall within any

established cataloging unit boundaries based on their latitude and longitude.

6. Using nutrient ecoregion GIS coverage provided by USEPA, all station locations with

latitude and longitude coordinates were tagged with a nutrient ecoregion identifier (nutrient

region identifiers are values 1 - 14) and the associated nutrient ecoregion name. Because no

nutrient ecoregions exist for Alaska, Hawaii, and Puerto Rico, stations located in these states

were tagged with “dummy” nutrient ecoregion numbers (20 = Alaska, 21 = Hawaii, 22 =

Puerto Rico).

7. Using information provided by TVA, 59 station locations that were marked as ‘stream’

locations under the “Station Type” parameter were changed to ‘reservoir’ locations. 

8. The nutrient data retrieved from STORET were assessed for the presence of duplicate data

records. The duplicate data identification process consisted of three steps: 1) identification of

records that matched exactly in terms of each variable retrieved; 2) identification of records

that matched exactly in terms of each variable retrieved except for their station identification

numbers; and 3) identification of records that matched exactly in terms of each variable

retrieved except for their collecting agency codes. The data duplication assessment

procedures were conducted using SAS programs. 

Prior to initiating the data duplication assessment process, the STORET nutrient data set

contained:

41,210 station records

924,420 sample records
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-20 Appendix C—Quality Control/Quality Assurance Rules

• Identification of exactly matching records

All data records were sorted to identify those records that matched exactly. For two records

to match exactly, all variables retrieved had to be the same. For example, they had to have

the same water quality parameters, parameter results and associated remark codes, and have

the same station data item and sample data item information. Exactly matching records were

considered to be exact duplicates, and one duplicate record of each identified matching set

were eliminated from the nutrient data set. A total of 924 sample records identified as

duplicates by this process were eliminated from the data set. 

• Identification of matching records with the exception of station identification number

All data records were sorted to identify those records that matched exactly except for their

station identification number (i.e., they had the same water quality parameters, parameter

results and associated remark codes, and the same station and sample data item information

with the exception of station identification number). Although the station identification

numbers were different, the latitude and longitude for the stations were the same indicating a

duplication of station data due to the existence of two station identification numbers for the

same station. For each set of matching records, one of the station identification numbers was

randomly selected and its associated data were eliminated from the data set. A total of 686

sample records were eliminated from the data set through this process. 

• Identification of matching records with the exception of collecting agency codes

All data records were sorted to identify those records that matched exactly except for their

collecting agency codes (i.e., they had the same water quality parameters, parameter results

and associated remark codes, and the same station and sample data item information with the

exception of agency code). The presence of two matching data records each with a different

agency code attached to it suggested that one agency had utilized data collected by the other

agency and had entered the data into STORET without realizing that it already had been

placed in STORET by the other agency. No matching records with greater than two different

agency codes were identified. For determining which record to delete from the data set, the

following rules were developed: 

< If one of the matching records had a USGS agency code, the USGS record was retained

and the other record was deleted. 

< Higher level agency monitoring program data were retained. For example, federal

program data (indicated by a “1" at the beginning of the STORET agency code) were

retained against state (indicated by a “2") and local (indicated by values higher than 2)

program data. 

< If two matching records had the same level agency code, the record from the agency with

the greater number of overall observations (potentially indicating the data set as the

source data set) was retained.

A total of 2,915 sample records were eliminated through this process. 

As a result of the duplicate data identification process, a total of 4,525 sample records and 36

individual station records were removed from the STORET nutrient data set. The resulting
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-21

nutrient data set contains the following:

41,174 station records

919,895 sample records
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-22 Appendix C—Quality Control/Quality Assurance Rules

APPENDIX B. Process for Adding Aggregate Nutrient Ecoregions 

and Level III Ecoregions

The flag_id tracks the type of changes that were made to the data. There are a total of eight flags

that are used to describe the changes made to the data. The flags are defined as follows:

1—The latitude and longitude coordinates match the county that was provided. If the HUC was

null, it was updated based on the latitude and longitude coordinates. The ecoregions were

determined by using the latitude and longitude coordinates.

2—The county and HUC are available, but the latitude and/or longitude coordinates are missing. 

Therefore, the centroid of the intersection of the county and HUC was used to determine the

ecoregions and the latitude and longitude coordinates. If the HUC and county did not intersect, 

the county centroid was used to determine the ecoregions and the latitude and longitude

coordinates.

3—The county is available, but the HUC and the latitude and/or longitude coordinates are

missing. Therefore, the county centroid was used to determine the ecoregions, HUC, and the

latitude and longitude coordinates.

4—The HUC is available, but the county is not and the latitude and/or longitude coordinates are

missing. Therefore, the HUC centroid was used to determine the ecoregions, county, and the

latitude and longitude coordinates.

5—The county is missing, but the latitude and longitude coordinates are available. Note: A

county is considered missing if it is invalid. In other words, if the county entered did not exist in

the state, it was considered null. Therefore, the latitude and longitude coordinates were used to

determine the ecoregions, county, and HUC (if it was missing).

6—The latitude and longitude coordinates did not match the county that was provided, but they

did match the HUC. Therefore, the county centroid was used to determine ecoregion values. 

7—The latitude and longitude coordinates did not match the county or the HUC that was

provided (including null HUCs). Therefore, the county centroid was used to determine

ecoregion values.

8—The latitude and longitude coordinates were missing, but the ecoregions were provided by

the state.

The ecoregions provided by the states were used as the ecoregion values.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-23

APPENDIX C. Glossary

Coefficient of Variation - A measure of variability. The standard deviation divided by the mean

multiplied by 100. 

Maximum - The highest value.

Mean – A measure of central tendency. The arithmetic average.

Median – A measure of central tendency. The value which cuts the distribution in half, such that

half of the values are above the median, and half of the values are below the median. Also called

the 50th percentile or middle value.

Minimum - The lowest value.

Standard Deviation – A measure of variability. The square root of the variance with the variance

defined as the sum of the squared deviations divided by the sample size minus one.

Standard Error - A measure of variability. The standard deviation divided by the square root of

the sample size.

5th % - the 5th percentile

25th % - the 25th percentile, the first quartile.

75th % - the 75th percentile, the third quartile.

95th % - the 95th percentile